- "My mind… is my own again. I cannot expect forgiveness..."
See Kain's Canon History at the FF Wiki for full details on his background. He is taken from Mount Ordeals at the end of the game, a matter of weeks after he arrived there.
Kain is an aloof individual, introverted about personal matters. Yet he's also extremely proud of his heritage and nation; this pride makes him come across as rather arrogant at times. As a child, he often bragged about his heritage, and his role as the fabled Dragoon's son has stayed with him ever since.
He isn't always an easy person to be around, and forms deep grudges. He's prone to dwell on issues, going into long periods of brooding or reflection. He can be cruel, and must always watch himself to keep his emotions in check. He is known to taunt enemies when in a dark frame of mind. This sadistic side is usually kept well under control (but thanks to Golbez/Zemus stirring it up in the past, this is a trait he can't always ignore). He fears being controlled or used as he was before, thus has grown much more suspicious of strangers' motives. Despite all of the precaution, his darker self is still not entirely beaten yet, and it is a constant struggle for him to contain it.
Jealousy is Kain's worst characteristic, making him obsessed at times with 'proving himself' against impossible standards. He constantly makes comparisons of himself against others, attempting to surpass them... which often frustrates him. If faced with a challenge he stubbornly won't back down; he has been described as 'headstrong'. Fighting and training are how he spends most of his time. He's known to often take off on solitary trips for his own purposes.
Often, it's hard for others to understand the real motives behind his moods. He keeps it all to himself until it blows up in a bad way. This repression makes him seem very indifferent on the surface. Still, Kain is a highly compassionate person underneath this exterior, and strives to protect his closest friends. Indeed, it is often for their sakes that he hides how he really feels, so that he doesn't hurt them. He will defend loved ones without question when need arises- especially Cecil and Rosa. He has a strong sense of loyalty and honor for those who have earned it. He's deeply remorseful for his past actions, something which is never far from his mind.
Physically, Kain is very strong. He's a Dragoon, a knight class with a jumping ability. He spent his childhood and onward training in this fighting style. It's also implied that Dragoon skills are inherited, so it's likely he has a natural aptitude for it. The air is Kain's element, and he dominates strongly over enemies of the sky. Jumping to great heights wielding heavy weaponry gives him an advantage over opponents on land as well. When he jumps, it allows him to evade attacks. He's excellent with quick, agile reactions- jumping as he does requires well-honed reflexes. He is a fierce, determined warrior, and stubborn about winning his battles. In game he even challenges Cecil to a duel, to prove himself better in combat. His preferred weapon is the spear. He has passable skill with a sword, and is able to hold his own in a fist fight (in a flashback in the updated game, he threatens to beat Cecil up as a kid).
Mentally, Kain puts his duty as a fighter above everything else. Since he was in a commanding position, this means he has familiarity with battle tactics and strategies, as well as a leadership role. He's also a loyal companion, despite the problems in his past. He spoke up to defend Cecil against the false king, and even while under mind control, had an underlying need to protect Rosa. Once a person has won his trust, he will support them through anything. He is also quite confident and prideful, with a strong sense of honor. Adhering to this honor is important to him, because it was his father's way as well. His stubborn streak allowed him to take a path he wanted, following in his father's footsteps rather than compromise and do as the King wanted. He is extremely confident on the battlefield.
Emotionally, Kain keeps a great deal to himself, making him seem to be strong in this way... at least, on the surface. He has the hardened attitude of a warrior. This can be a real advantage in a crisis. When there is a problem, he takes it inside, not complaining or otherwise seeming to be bothered. He does what is required of him because that was how he was raised and trained to be. His pride would never allow him to willingly appear weak. He doesn't back down from challenges, no matter the danger.
Physically, Kain is not innately skilled with magic, and therefore his offense and defense in this area are both very low. This leaves him very vulnerable to magical attacks. He's especially weak against mind control and possession, as shown by the ease with which Golbez/Zemus invaded his mind.
Mentally, he is therefore much weaker than he appears. His habit of keeping his thoughts to himself works against him more often than not. His prideful nature and tendency to isolate himself cause him problems. He's more likely to seethe and obsess about something than show it in any rational way. He is hard to figure out, even by his closest friends. This can be a real disadvantage, because he is much less stable mentally and emotionally than he appears to be (these weaknesses are linked closely together- his emotional flaws essentially weaken him mentally). His secret instability leaves him vulnerable, and easily manipulated if he isn't careful. He's deeply ashamed by his mental weaknesses in particular. Thus it is something that he won't be open about, which he hasn't confronted or worked through.
Kain also has a reputation for being a little too merciless toward his foes; during the attack on the village of Mist he was willing to even kill Rydia to carry out the King's orders. This can make him seem quite intimidating and unapproachable. He also taunts those he holds a low opinion of... especially when in a bad mood, or all out evil mode. For instance, he makes snide remarks at Cecil when their paths cross, while he's serving Golbez/Zemus. He generally seems condescending toward many of those he doesn't already consider a close friend, too- notably Edge when they first meet him. This trait is definitely more likely to make him enemies than friends.
Emotionally, Kain is extremely unforgiving of faults, especially his own. He's tormented by his guilt for past actions, which certainly affects his emotional state. Kain has trouble handling his feelings, and will often bury them entirely until they boil over in an unhealthy explosion. This is true of his anger and other negative emotions, but especially the case for jealousy. Jealousy has always been Kain's greatest downfall. Indeed, it was this flaw which ultimately allowed Golbez to gain control over his mind. He also has a fondness for Rosa, which he prefers to remain quiet about. Even so, these feelings can be used against him. His need to prove himself to someone stronger- perhaps rooted in a longing to replace his lost father-figure- also accounted for the ease with which he was manipulated. He seeks approval from others a great deal more than he lets on.
Kain lives with a constant struggle against his dark side. This amounts to a battle against emotional and mental darkness. In the game, he becomes almost a different person under Golbez's power- speaking his mind more, and being even more abrasive than usual. This side must literally be fought in his world, to be overcome. Because he has not done this task, it remains part of him... rather like a ticking time bomb waiting to explode.
- "The fact remains that I've by no means redeemed myself..."
Cecil – Kain's BFF from childhood, Cecil is someone who Kain respects greatly and also holds insane amounts of jealousy toward. He feels immensely guilty about betraying him, despite Cecil's insistence of forgiveness.
Rosa- The unrequited love interest. Kain has also known Rosa since childhood, and his admiration for her became a strong crush. Unfortunately for Kain, she is married to Cecil. He hasn't done well in getting over her.
Rydia- Someone else to whom Kain holds lingering guilt, considering what he and Cecil did to her home and kin. Yet he's been able to form a friendship with her anyway, becoming accustomed to her habit of visiting when he isolates himself. He trusts her, surprisingly finding her to make a great deal of sense with her directness.
Edge- Kain is never certain where he stands with Edge, thus he's always especially guarded around him. He still finds himself often rolling his eyes at Edge's mannerisms, not to mention his bold attitude. Though even Kain can't disregard his skill in battle.
Golbez- Formerly an evil master he bowed to, Golbez presents yet another dilemma for Kain. On the one hand, Kain realizes that Golbez was used just the same as he was, and sees Cecil starting to accept him. He knows that it would be irrational to hate him on principle. On the other... Golbez is a reminder of the horrific past, someone in whom Kain sees some of his own failings reflected back at him. He can't help but hold some resentment toward him.
Valvalis- Kain's sworn enemy. Perhaps her affinity for his favorite element adds to his hatred, perhaps also her personality... whatever the case, she's the one out of all the Elemental Lords who he's always found himself most at odds with, even back when working under Golbez/Zemus' control. That hasn't changed with a new location, and he is constantly waiting for chances to strike against her.
Rubicante- Another individual who Kain would sooner see destroyed. He doesn't like the way Rubicante- or any of the Fiends- willingly follows Golbez. He can't bring himself to trust him, even if he shows a more honor-bound side at times than some of the others.
Seifer- An enemy that Kain made in Luceti, Seifer has earned Kain's wrath on multiple occasions. Ironically, there is some similarity between the two with their stubborn, prideful attitudes... not that Kain would ever acknowledge such. He just finds him obnoxious.
- Not listed on here? Want to know what Kain thinks of your character if they've interacted? Please let me know! I'll edit this section periodically.