Sokuochi Mayuri-chan - Vol. 1 Ch. 15 - You said that you would obey anything right?

[Al_Kohlick]

saitama taught him well

Indeed. I've been thinking for a while that this guy deals with his GF's shenanigans like Saitama deals with monsters : with a deadpan face and one killing blow.
And now, look on page 2 : he has even borrowed Saitama's signature 'Oppai' sweater. There are no coincidences.