If I ever saw a picture that needs a What tag, this is it.

Not really. Parent post shows Miku brushing Mikudayo's hair so this post is Mikudayo returning the favor but making a clumsy job of it.

Posts are also a reference to Tokyo Game Show 2019 where both Mikudayo and Miku appeared in costume together. [1] [2]

EDIT: The other character actually has a name and is called Mikunano.