Dustfeather
This is a really lovely idea! I adore flowers, so this sort of thing is always really interesting 
Had a couple of thoughts as to which flowers might fit which characters:
307 zinnia // i mourn your absence
Thought this one might be fitting for Princess, as apparently one of the Erins stated that she still finds herself looking out for Firestar when she's out and about, hoping she'll see him again (one of the more heartbreaking authors' comments, in my opinion :c)
26 bellflower // gratitude
Littlecloud was my thought for this one; his inspiration for becoming a medicine cat was his gratitude to Cinderpelt, and subsequently ThunderClan as a whole, for saving his life 
10 alyssum // worth beyond beauty
Brightheart perhaps? 