Family Psychology in Kingdom Hearts
Aside from being the prequel in the series, Kingdom Hearts: Birth By Sleep essentially centers around two surrogate families, which were both broken apart by Master Xehanort. The first one is headed by Master Eraqus in The Land of Departure. The second is comprised of Ansem the Wise and his apprentices in Radiant Garden. Xehanort's status as a home-wrecker has not been fully explored (overshadowed by his more important role as main boss), but analysis of these forged bonds among the members of these families will show how much of an impact he really has on the group dynamics of these characters.
Eraqus starts off with Terra (who is described as being like a son to him) and Aqua (no relationship given but she is like a sister to her fellow apprentices Terra and Ventus). Xehanort then dumps his own pupil (post-darkness-op) Ventus into Eraqus' lap. Terra is all too excited to welcome the new kid, but the barrage of basic questions overwhelms the amnesiac Ven, who begins to scream in pain. As is the case with all older siblings, Terra gets reprimanded by Eraqus for upsetting him, though this was not his intention. This unofficially sets up the running issue that Terra gets blamed for everything that goes wrong in the game whether or not he's actually guilty. Still, Ven looks up to Terra and they get along like most brothers do; Aqua gets along with them like most girls with no other women in their lives do.
Eventually, despite Xehanort's efforts to drive them apart (and Eraqus' being a colossal dumbass), they are still united in their final fight against him. Before the game's end, however, they are fractured and become separated from each other for several years. It also doesn't help that Xehanort literally stabbed Eraqus in the back during an argument with Terra about whether or not Ven's life should be sacrificed in order to stop Xehanort's plan. Eraqus even knew about what his friend was planning but didn't actually think he would go through with it so he didn't warn the apprentices. Thus, this family's dysfunctional nature centering around trust issues became exposed pretty quickly. Xehanort forcibly took possession of Terra's body, but both his and Eraqus' spirits are shown fighting over Terra's soul in the Keyblade Graveyard where Terra's lingering sentiment is waiting inside his armor.
While going through a period of amnesia in Terra's body, Xehanort infiltrates another family in Radiant Garden headed by Ansem the Wise. His surrogate family is made up of his apprentices: Braig, Even, Dilan, Aeleus, and Ienzo, the latter of whom he'd adopted. Fan speculation puts one or both of Ienzo's parents under the employ of Ansem as well before their deaths (possibly caused by illness or accident, but no details are provided). Due to the wide variance of opinion, the family dynamic here will always be interpreted differently. For example, some fans think that Ienzo was always an evil little boy (though no one has gone so far as to accuse him of actually causing his parents' deaths) and characterize him as being as nefarious as Xehanort in regard to the events leading up to the first game. I personally disagree and venture to present evidence to the contrary.
For some reason, Even feels the need to exposit that Ienzo is adopted to a total stranger (Ven). This seems to suggest that his parents' deaths had happened in the recent past (otherwise, why bring it up?). The boy doesn't speak and has "eyes without fear" when staring down the Unversed, which do nothing but stare back at him (which also suggests that fearing or fighting them only makes them more dangerous and violent). This is more likely to be indicative of recent trauma rather than underlying malcontent. When Even is outside looking for him, he breathes an audible sigh of relief when he sees the boy come running after his name is called. However, while he was calling his name he didn't sound markedly panicked. He was probably relieved that the boy was no longer lost and he in turn wouldn't be in trouble with Ansem (nice job, babysitter). He then seems to badmouth the kid for wandering off despite their best attempt at raising him. Ienzo makes no effort to defend himself like he does when he's older (Vexen still talks over him and treats him like an idiot until threatened by Lexaeus). We later see him happily eating ice cream with Ansem before looking at Xehanort and Braig suspiciously (also note that Ansem is holding his hand, while Even never does this; Even probably has childhood issues of his own).
Even also tells Dilan and Aeleus what to do (earlier he'd told them not to leave their post while Ven ran after an Unversed, and they obeyed; I'd like to think they also took the opportunity to ask him was his young charge was, prompting his search). The two of them are shown to be fiercely loyal and protective, but not much else is confirmed to be known about them personally. Braig can also tell them what to do, so I'm guessing the red bandana he wears signifies that he's the guard captain. It's also likely that Even (an evolutionary biologist at most and chief medical officer at least) was the second-in-command of the facility before Xehanort rose so high in Ansem's favor. Braig was Xehanort's first lackey, and his influence soon spread to the others as well. When Ansem finally got wise to what was going on and wanted to put a stop to Xehanort's experiments, however, he wasn't killed like Eraqus was, merely banished. Why? Terra's heart is still partially awake, so he wouldn't condone the murder of another father figure; it's even less likely that the other apprentices would have been on board with that. Banishment must have seemed like a reasonable compromise at the time.
Family is still somewhat important after they become Nobodies, though not in the way it was before since they believe their hearts have been lost to them. Organization XIII, in some respects, is another big dysfunctional family headed by Xemnas. Zexion makes a comment that the Organization is splintering and that it was the bond that held them all together. This indicates that he still thinks of everyone as family even in this form. While this can also be interpreted as him just wanting everyone to get along so the plan can come to fruition smoothly, this proves to be wishful thinking as more backstabbing occurs. We as fans can only hope that Kingdom Hearts III, the end of the Xehanort Saga, will finally put everything to rights.
Highlight blank space to reveal what were considered spoilers at the time.
(Now, the light novels are a source of some confusion on my part. From the incomplete translations I've read, Zexion thinks to himself that he's always hated Vexen, even when they were people [whereas Xigbar has always been unbearably annoying]. There is also another passage that suggests Vexen was about to slap him before Lexaeus stops him. That is a surprising sentiment and equally surprising behavior, given that Even's character box states that he "treasures Ienzo above all others." Then again, Ansem loved them and in his anger created a war against all Nobodies [which is also sad given that the original six still refer to themselves as "that person's beloved apprentices"]. But what could have gotten those two so mad at each other? Before Vexen almost slaps him, all Zexion said was that neither of them had greeted him before asking him what had happened. Also, there may be some mistakes in the writing. After all, it is Xaldin who is shown to be manipulative, not his true self, yet that's what appears in Dilan's character box. We also don't know what was going on between them during their appearance in the end credits in which Even walks away, then Ienzo sighs and hurries after him. There are fan theories that suggest Even had accidentally killed Ienzo's parents in an experiment, but I'm not apt to believe that either [nor do I believe that Vanitas was responsible].)