Leon appears to have sunflowers behind him, they have a lot of varieties and his looks like some of the smaller subspecies

Sunflowers are often associated with joy which we don't get a lot of from Leon, but they're also known to represent hope, resilience, and loyalty which are absolutely through lines in Leon's character arc

Claire looks as though lilacs are meant to represent her

Lilacs of this color usually represent first loves which I didn't initially feel represented her because she has no canon love interest but they also have an association with new beginnings because of how early in the year they bloom— her entire arc in RE2 is about how her life changes forever. They also are sometimes used for protection and can symbolize that and Claire's entire character is about her being a protector

Chris is undoubtedly gladiolus, also known as sword lilies because of the shape— the name coming from the Latin word for sword, gladius

The gladiolus generally represents strength, integrity, morality, and faithfulness. I don't think I need to explain how that represents Chris. Interestingly, the pink ones specifically represent admiration, subdued love, and maternal affection. I do think that maternal aspect can show in how he raised Claire after their parents passed but with the admiration... maybe Wesker if the Uroboros tentacles and heart behind him are anything to go by?

Jill has quite a distinct flower in the lily of the valley

However, this did confuse me since I knew lilies of the valley to represent innocence and purity but after some light reading I thought we could take that to mean moral purity since I also found out it can represent humility and more importantly, virtue. On top of that, the original meaning of this flower was a "return to happiness" which I don't feel we've really seen in Jill's character arc yet. However, RE9, a game all about the past has yet to release and who knows? Jill could play a role

Grace was harder for me since we don't know much about her yet but I landed on white lilies

Similar to the lily of the valley, they represent purity and innocence but ALSO new beginnings, the idea of Requiem. (maybe we really will get to see Jill again please please please please) White lilies are also the most traditional flowers for a funeral so they're fitting since Grace's arc is going to revolve around her late mother. Even better, they're also known to represent motherhood, long lasting love, and protection. So maybe we'll see Alyssa playing a role from the grave in protecting Grace

Ada is last but never least. Her flower is clearly a thistle

Thistles can grow in very harsh conditions and as such are often used as a symbol of strength and perseverance as well as bravery and determination which I think goes very well with Ada's personality. Their prickly but pretty look is also used as a symbol of the beauty in roughness which captures Ada's femme fatale personality very well. It's also possible that the "leaf" next to the thistle is actually rosemary which when paired with thistle was a way to signal to someone that you could see through their façade which is goddamn perfect for Ada