I felt Terra's arc had a lot of depth. Initially, Terra is lost and confused, doesn't view herself as a leader, and needs others to protect and save her. Furthermore, due to debilitating effects of the slave crown (which controlled her mind and suppressed her emotions), she has a constant fear that she lacks the ability to feel. In particular, she's frightened she lacks the ability to love, the emotion she wishes to experience the most. (The player is led to believe this refers to romantic love, since that is how love is portrayed the overwhelming majority of the time in media). As the game progresses (esp after she regains her memories), Terra becomes more confident in herself, and takes the initiative to face the empire. Unfortunately, things don't go according to plan- Kefka places the Warring Triad out of balance, and the world gets destroyed. In the post-apocalyptic world, Terra wanders to Mobliz, where she finds orphaned children whose parents were killed by Kefka's "light of judgement." Terra sees that the village is falling part, and needs a parental figure to hold it together. Out of necessity, Terra emerges as a strong leader, and becomes an adoptive mother to the children. Here, she begins to feel strong emotions that prevent her from fighting. Eventually, she realizes that this feeling is love- she finally has closure in knowing she can feel, and that she knows what it's like to love and be loved. What I loved is how this is done in a very unique and non-cliche manner. Terra's finding of love isn't romantic: it takes the form of motherly love. Terra is content with it being this way, showing how strong and mature of a character she is.
I will admit, the premise of her character, a lost, confused, dependent person suffering from amnesia, isn't particularly unique (though it was used less often back then). Neither is her development into a more confident and independent person super unique as well. (Not to say that this is bad- it was pretty good and made her a three-dimensional character, but it wasn't super innovative). What's brilliant about her story, however, is how well it was executed. And it was executed in a very unique and non-cliche way. Her finding motherly love instead of romantic love was a huge curve-ball to the audience...and it exemplifies one of the most profound themes of the game: love can manifest itself in many ways. Terra is a very good three-dimensional dynamic character, whose character growth really resonated with me. I also liked how her arc didn't revolve about being "trapped in the past," something that Locke, Cyan, Strago, and Shadow suffered from. Again not saying being "trapped in the past" is bad or uninteresting, imo, it was done will with Cyan and Locke, it's just something that wasn't very unique. The fact that Terra's story diverges from that model makes her story refreshing and stand-out.
Another character I loved was Celes. Her story arc was very profound, but in a different way than Terra: Celes was a foil to Terra, paralleling her in character development.
It's pretty obvious that Celes was a foil to Terra due to their different battle abilities: Terra's natural element is fire and Celes' is ice, and Terra's stats focus on offense where Celes' focus on defense. But their story arcs parallel each other in opposite directions, with each receive the most positive outcome in the end, but in different directions. Celes starts out as very stoic and fiercely independent. She presents herself as noble, tough, and dignified. When Locke first meets her, she tells him she prefers to be executed for her actions, since she would retain her pride. When Locke insists on rescuing her, Celes is confused. She's rarely experienced altruistic acts of kindness (although Locke's true motivations were a bit more complex). When people usually interact with Celes, they treat her as "Celes the general." not Celes the person. As a result, Celes presents herself with a tough exterior shell by not being emotive. Unlike Terra, Celes knows she has emotions, but chooses not to express them because she has this military mentality that "emotions are for the weak," (In contrast, Terra wants to feel emotions, but is scared she doesn't have them). Celes also starts out with somewhat of a superiority complex: when Terra innocently asks Celes whether she can feel emotions (Terra asks Celes b/c Celes too underwent magical experimentation and was bred by the empire), Celes takes Terra's question the wrong way and as an insult. (Celes interprets it to mean "are you mocking me, by thinking that because i'm with the empire, i can't feel emotions? and you feel that not showing emotions is somehow wrong?).
However, as the story progresses, Celes starts warming up to the rest of party. By being so nice to Celes, Locke manages to break away at her tough exterior; Celes willingly opens to up to him because for once, someone is making an effort to get to know the actual person she is. Establishing a relationship with the other party members is a bit more difficult, since they are suspicious of her because she used to be an imperial general. But as the story progresses, and as the characters interact with each other, and Celes takes the lead in some schemes (like the airship scheme), the other characters become her friends. Basically, while Terra becomes independent, Celes learns how to be co-dependent, and learns it's not so much of a bad thing to ask others for help from time to time. She leans it is okay and only natural to express emotions, and the beautiful opera scene (at least the SNES translation), is a really humanizing moment for her: her lines when she throws the roses off the balcony (which shares the same animation as when Celes' attempts suicide) shows that her real self fears of being alone.
Her exterior has more or less worn off by the Magitek Research Facility segment- that's why she's so emotionally distraught when Locke doubts her loyalty...someone she opened up to and trusted questioned whether she was on his side. She and Locke make amends when it seems like the war is over, and when they realize they have been tricked, they decide to confront Emperor Gestahl and Kefka on the same side. When the world gets destroyed, and if Cid dies, Celes assumes that she's alone in the world. This is an extremely emotional moment in the game, and reveals a lot about how Celes actually is. Deep down, all along, she was someone that felt alone and wanted to be in others' company. She thinks her friends are dead, she thinks Locke, one of the first people to genuinely wanted to know her, is dead.
The reason she is extremely happy when she sees Locke's bandanna after surviving her suicide attempt is because she has hope that Locke and others are still alive. She wants to go and find the man she loves, and the friends to whom she has become close. Therefore, It makes perfect sense that she leads the effort to find and regroup the characters in the WoR. Very strong character development, and great contrast to Terra.