Their designs are fine, no more or less elaborate than what you'd expect for a Pokemon trainer. The simplistic clothing makes them more grounded and relatable - because anyone can pull out a pair of shorts and a jacket and pretend to be a Pokemon trainer - while the hair colours give them a more outlandish vibe expected from an anime. The red and blue colours are an obvious reference to Scarlet and Violet, and likely convey a difference in personality.
We can gleam parts of their personality from their poses and how they're arranged on the poster. Liko is in the forefront, facing the camera with an open pose, suggesting she's honest, confident and outgoing. The likely plot-relevant shard hanging around her neck is on full display. Roy, on the other hand, is slightly in the background, looking over his shoulder, suggesting there might be more going on with him than we might think. He has this vibe of quiet confidence.
So I guess my expectations are that Liko will be the go-getter newbie, while Roy will have slightly more experience which will tell in the early days.