Warning: Spoilers are scattered liberally throughout this entire application.
Personality: Emil has two personalities. First, there’s Emil Castagnier, who has all the memories and personality of a shy but mostly normal kid who’s been tossed into a situation that’s made him grow rather quickly. Then there’s Ratatosk Mode Emil (also known as simply Ratatosk), who has the memory and personality of a millennia-old spirit who’s been betrayed, killed, and turned amnesiac only to suddenly remember who he was again. They don’t always get along.
Emil’s not a confrontational person. Given the choice he’d rather avoid unnecessary conflict and will go along with others unless he really needs to take a stand or has a purpose in mind-when he has a goal he’s more willing to push. Occasionally he’ll apologize for things when he doesn’t have to, and he sometimes feels responsible for things he has no real control over: once he finds out he’s Ratatosk he feels like such a traitor to his friends that he stops writing in the journal he shares with Marta for a while and later tries to deny himself a normal life twice for guilt over Aster.
However, his motto is his inspiration: "Courage is the magic that turns dreams into reality." It comes from Richter, who made a big impression on Emil when they first met, and since then when Emil’s been afraid to do something he needs to do he’s used it to give himself a little extra courage and even to encourage others. There’s other lines that he draws upon from time to time too, including "Are you a dog? Or are you really a man?" from Richter again and "Come on, you’re a man! Speak up!" from Marta. (He’s also poked fun at the latter line-Emil has his occasional sarcastic moments, generally when he’s miffed or with friends.)
Emil has his not-so-bright moments, occasionally reading into things too much or too little, or stumbling over himself for other reasons. He’s not book-smart either, as most of his schooling comes from inconsistent false memories. It’s in personality and spirit that Emil shines: he’s a nice, sweet kid who cares about his friends dearly. After six months of a town that hated him and hating/fearing the town back, Emil really appreciates the friends he’s got. He’s a fairly easy read to anyone perceptive enough to read him-when he gets it in his head to sacrifice himself later on no less than three people in the space of two days figure out he’s planning something-but he can be perceptive and understanding himself as well. He judges his friends based on who they have been with him and has taken major revelations over their pasts with complete trust, though he can have his prejudices (e.g., Lloyd, sometimes the Tethe’allans).
He’s very loyal and will do anything to protect his friends. Especially important to him are Marta (with whom he has a developing romance) and Richter (whom he looked up to and has made some…adoring comments over), though he feels he’s done wrong to both of them in some way. He also cares a lot for others, friends and strangers alike. This isn’t to say he’s all nice guy, though: early on in the game when he believed Lloyd killed his parents, Lloyd was pretty much his Anger Button for a while and Emil even writes in his journal that he can feel his blood pressure rising at just the thought of Lloyd. He was a lot saner when confronted with the real murderer, though his friends had to convince him to skip revenge for business.
And that said, Ratatosk. If Emil is Ratatosk’s kindness, you could say Ratatosk Mode Emil is Ratatosk’s ruthlessness or anger (what’s shown of the "whole" Ratatosk isn’t entirely like either of the two, so it’s possible each personality was somewhat polarized). Ratatosk doesn’t care as much what he has to do to regain his former power and resume his duties. He looks down on the other Emil, considering him a coward dependent on Ratatosk’s power, and has in the past made a few other derogatory remarks about his "weakness" and "cowardice."
It’s all justified: Emil really did used to be a coward, and now he borrows Ratatosk’s power and never fights himself. Despite that, people tend to cast Emil as The Nice Guy while Ratatosk is The Jerk. And Emil is nice and kind, which also reeks of weakness to Ratatosk. Being strong in both combat and mentality is important to him, and in his opinion Emil doesn’t cut it. (This isn’t to say his opinion won’t ever change, however. It’s a matter of standards. Later on when Emil decides to sacrifice himself and gains the resolve to seal Ratatosk’s personality away, he’s shocked. The next thing he knows Emil’s managed to stop Richter and has decided to face his problems head-on, something Ratatosk probably hadn’t considered him capable of before, and Ratatosk ended up acknowledging Emil at that time.)
Ratatosk’s not big on people: humans and half-elves waged a war that destroyed the tree he guarded, which wreaked havoc on the mana supply he controlled. He trusted a young half-elf, who forced him into taking a four-thousand-year dormancy by splitting the world in two. Not exactly a good impression. On the other hand, considering he originally meant to kill mankind and started by killing an unlucky researcher for daring to speak up, he’s improved. Ratatosk too has been impacted by Emil’s travels, and that’s tempered his wrath.
It is possible for Ratatosk to have his nice moments. Not frequently, as he prefers to be more direct and to-the-point, but he’s pretended to be the other Emil to help Marta despite his resentment and later he’s concerned about civilians’ safety after she tells him to stop being so careless about others’ lives. He probably wouldn’t try to destroy mankind again given the chance-but he won’t be eager to meet new people either.
The revelation that Emil is Ratatosk has also changed things, because now Ratatosk’s personality is the one with the memories and the knowledge. Canon’s not clear on his reaction, but from what is shown he has, in fact, regressed in part to his old personality in response to recovering his memory. He threatens to take down anyone who gets in his way all for his objective of keeping the door closed, as the fact that he’d be fighting Emil’s friends (even Marta, whom he’d protected as a Knight of Ratatosk) doesn’t compare to the consequences of letting demons run amok and he’s an "end justifies the means" kind of guy. He’s determined to restore himself to full strength and do whatever he needs to in order to fulfill his duty is done and keep the world safely in order.
While Ratatosk observes what happens to Emil at all times, whether he or the "normal" Emil is in control, the "normal" Emil usually remembers what’s happened after switching back from Ratatosk Mode only vaguely at best, though there have been one or two incidences where he fought Ratatosk’s control and many more where he just switches back when prompted so it’s likely that he’s sort of aware while in Ratatosk Mode but tends to forgets after the switch.
It should be emphasized that both Emil’s and Ratatosk Mode’s personalities are both Emil and both Ratatosk, just two sides of one coin. One of the major points of the game is how people don’t realize this fully and instead sometimes treat them as different people, only to get reminded that both Emils are Emil. They aren’t entirely separate people, even if there isn’t much trust between them right at this point. Both have had the same experiences as Emil Castagnier; both of them have the same friends; and even if Emil’s still unsure of himself at this point, both of them have the same task to uphold as the Summon Spirit Ratatosk.