Layna: Very plot-important, honestly. She's an over-arching character (to avoid spoilers). It's more apparent in the Demon Path, but she does have this all explained in the normal path, so it's not limited to the one path. She is an excellent combat character and makes a great leader (high stats everywhere - gamebest overall I believe, more than Revya without absorptions - that increase with the leader adjustment bonuses by a lot in addition to great field tactics and a powerful special attack). She is late, but it really strikes me as more of an Athos case than anything.
Vitali: Lots of plot interaction. Arguably, he's the least likely to be used as a leader due to poor field movement. His field tactics and special are pretty good, but most people won't see his attack (although they'll know it exists), usually putting him in the back to heal. Not that people won't know what he does when he fights - that's see with generic clerics as well. It's more the he does not translate at all (which, again, isn't abnormal), not that he's present as a plot character.
Tricia: In-between. Her plot relevance is there definitely; not as much as Layna or Vitali, but still there. Unlike Vitali, she actually is a good squad leader, and will likely be used for her attacks (as archers have a great middle attack). He special is also something likely to be seen a lot, due to her range and low STM. So she's definitely a good option based on gameplay and plot relevance, and is present at least half of the game (and has a lot of Shauna connections, who is the most present boss in the game, so that helps too).
Incidentally, can you explain (Mr. Elf), how this works? I've argued before for a 4-stage ranking schema (and will likely continue to argue it after this), where it's
1) Choose the games
2) Choose if the games make it or not
3) Choose which characters make it or not
4) Rank the winning characters
If someone abstains on a game but gives a list of characters to rank, I was told that their list of characters doesn't count. I've been told the only specific character ranks that count are those that also have a "yes" to the game as a whole (while a "no" to the game doesn't count for a no to each character individually). Can you provide a detailed explanation of how this whole process works, as I think it might help everyone if it were clarified?