This is another character from the old trailers who had relevance. This one's name is Tako-Chan. She is one of three. Now all she does is appear as a cameo and vanish when spoken to.
You might notice her design's a little different from the one most people draw. It seems that when making the jump from Omoriboy to Omori, Tako went through a design change to presumably reflect the change in the protagonists age. Originally she just wore panties and a very small shirt. Here she has a striped shirt and shorts/pants. It's also worth noting that one of the old concept art sheets showing the gender swap option for a few characters also included two designs for Tako. The sprite that appears here is the female one. You can tell because she has bunches.
Tentacled Haru cameo