She responded with "that's gross" because she's a Tsundere. The [I love you] is what she really means. She's bad mouthing him because she's embarrassed by how much in love with him she is.
words on his stomach are just onomatopoeia. literally its "byuru byu", and closest translation would be something like "splurt", since it's apparently the sound your dick makes when you cum.
This girl is good friends with the menhera girl. when she debut there I thought she was strict with him in the sense that she helps him improve his lot in life, not hurl headache-inducing tsundere insults