I think I identified all the characters, although I don't really know anything about Sakura Wars or Phantasy Star Online. The two characters in the lower right don't seem to have tags in Danbooru.
I don't know anything about that Ulala character, but Ringo does a nice cosplay of her in the game: https://puyonexus.com/wiki/PPQ:Ringo_ver._Ulala/%E2%98%856 To be honest, I think Ringo looks better drawn in the normal Puyo art style than Ulala does!
Knuckles
Sonic seriesLove
Oshare Majo Love and BerryErica Fontaine
Sakura WarsGemini Sunrise
Sakura WarsBerry
Oshare Majo Love and BerryAmamiya Sakura
Sakura WarsQuna
Phantasy Star Online 2Tails
Sonic seriesShinguuji Sakura
Sakura WarsClarissa Snowflake
Sakura WarsMochizuki Azami
Sakura WarsAnastasia Palma
Sakura WarsShinonome Hatsuho
Sakura WarsMatoi
Phantasy Star Online 2Shadow the Hedgehog
Sonic seriesMetal Sonic
Sonic seriesAmy Rose
Sonic seriesUlala
Space Channel 5Sega Collabo(ration) Gacha