I don't know what character it's supposed to be, but it looks a lot like the sprite used for the Ham-Chat word "Giftee" (Give) in Ham-Hams Unite:
This word was not used in Ham-Ham Heartbreak, but it's possible that it was originally planned to be used and the sprites were for whatever hamster (or hamsters?) would have taught it to you.
I think this is something that should be posted on The Cutting Room Floor's page for the game: https://tcrf.net/Hamtaro:_Ham-Ham_Heartbreak
Did you notice anything else that doesn't show up when playing the game?
Also, what did you use to explore the game's files? I might want to do it too
Am I the only one who has Hamtaro's sprites that have one present like this?