He can't be a buttercup because these have been in the Underground all along and he's supposed to be the first flower from the outside world that grew down there, a golden flower from Chara's village. Also Buttercups got five petals and he got 6. But what are these 'golden flowers'?
RpNation
Andshesaid
Andshesaid
You think they're rather wild yellow roses or Anglestem Primrose-Willow Ludwigia leptocarpa.
Grillby
or a yellow daisy....
Andshesaid
I never saw a yellow daisy though..
Grillby
Its still possible, also it would make sense if he was a Daisy since in the flower dictionary Daisy stands for death. ( Is a plant nerd)