Todomomo

Momo did it randomly one morning.

Kirimina

Kiri totally said it.

Miruhawks

I think Rumi might've let it slip once while they were fighting over something like one of them getting seriously injured. Hawks either teased her or cried when she said it. Most likely cried.

Iidamei

Mei said it first

Kamijirou

Kaminari totally said it

Dabi X Toga

Toga said it obviously.

Nejitama

Tam tam said it a few times and meant it platonically but then he said "I love you" at a formal event or sm.

Mt.Kamui

Kamui said it.

Ryushot

Hard one! I think Ryuko probably said it but he stood there for a moment and didn't say anything before finally saying it back.

HitoRei

Reiko said I Like You. Hitoshi said "I love you" when she cried once. He says I love you first a lot more than her. Their wedding, birthdays, Rosalind's birth, anniversaries. He usually says them first but Reiko might be fast and say it first.

TetsuKendou

Tetsutetsu said it first. Said it by accident.