Mostly For These Reasons Here - Tumblr Posts

people have been saying the mystery valentine isn't Gaster because the Japanese version is translated into hiragana + casual Japanese instead of the all-katakana + extremely formal Japanese that gaster usually speaks with in jp BUT .. here is why it is Gaster anyway, Actually

meta reference to Deltarune, The Game, which is something that only Gaster has done previously

comment about us "waiting patiently" (specifically in the context of deltarune) similar to the previous gaster posts on the undertale twitter account

"will you help me?" and "i will be counting on you." also very reminiscent of how gaster has talked to us before

the letter being "more or less completely illegible"

possible reference to error/placeholder text "You've come again to this wonderful place. Now tear off the mask beneath your face!"

bit about forgetting themself (or someone else...)

allcaps gaster text has been translated to Normal japanese text before for readability purposes:

as for why he's speaking informally it could just be because it's, well, a valentine's letter. every other example we have of gaster's speech, it's in a very serious/formal situation, but this is much more friendly and casual. basically he is just being silly, OK??