On Zelos and trust issues
For all that Tales of Symphonia's visuals look like something out of Candyland, with Meltokio possibly winning the award for most ridiculous city and most ridiculous inhabitants, Meltokio is not a great place. This comes up a couple of times in the skits, with Zelos and Lloyd remarking on the sharp gap between the rich and the poor, the very visible segregation (poor from rich, half-elves from humans, etc.). The nobles are made to look and sound ridiculous, which I think kind of downplays just how toxic the upper class is. It's an absolutely appalling environment for an orphan to grow up in, least of all the successor to a family as wealthy and influential as house Wilder is. In Zelos's EX manga chapter, a couple approaches him after his mother's death. They coo and say how unfortunate the whole thing is...and then invite him to a party. They invite a ten-year-old boy whose mother was just murdered in front of his eyes to a party.
These are the kind of people Zelos surrounded by throughout his whole life, so it's absolutely no wonder he grew up as incapable of trust as he is. The personality Zelos Wilder has so carefully crafted requires that he not do anything for others at the risk of his own emotional safety. In truth he's pretty sensitive, but he's hardened himself, and part of that means hardening his heart to others. He is selfish and rarely does anything unless he has something to gain from it; around the party his behavior is frequently annoying at best or borderline repulsive at worst, much of it with the express goal of keeping them from caring about him. If they care, they'll try to get close, and he can't afford that. Worse, Zelos can see just how much damage he's caused to himself, and loathes the person he's become; he's well aware that he's a dick, but the problem is that he can't see past that. He makes himself into a person who can survive without trust, but he can't actually live without it, and in Kratos's path that's ultimately what kills him. Lloyd asking if he can trust Zelos--essentially if Zelos is worth trusting--only affirms to Zelos that he isn't. Trusting in Zelos leads to (literal and figurative) dead ends, because Zelos really is that awful of a person.
So when Lloyd actually tells Zelos that he trusts him, the impact on him is immense beyond words. But while from that point on, he's committed to Lloyd--he makes that choice to commit himself to Lloyd's ideals--that trust terrifies him. The last person who trusted Zelos was Seles, and look at how that ended up: he's convinced she hates him because he ruined her life. He is terrified that he's going to fuck up so badly there won't be a second chance this time. That's what the Coward drags out of him, and that's why it takes Seles's form. For Zelos, that confrontation is about the two people who have come to matter most in his life because they think he's someone worth trusting. He's wedged between the only two people who have ever trusted or relied on him, and Mithos uses that to assert his ideology. The illusion's strength came from Zelos's fear that he would let Lloyd down the way he's let everybody else down.