When Gasper and Issei first met, Issei thought that he was another girl and was shocked after learning that he was a cross-dressing boy. Issei often reprimanded him for being a shut-in, although Issei and Gasper quickly became great friends and Issei trains him as his junior. Issei's blood gives Gasper a boost in power, even allowing him more control over his Sacred Gear for a short time. When Issei died in Volume 11, he, unlike the other members in the Gremory team, became very enraged, and defeated Georg afterwards after revealing his raw power.
Rias found him after his death and took pity on the boy, reincarnating him into a Devil through the use of a Bishop Mutation Piece. Rias has shown great patience with Gasper and his various discomforts, but later assigns Issei to help him get over them.
Kiba and Gasper seem to have a good relationship with each other, residing in the same house.
The first friend Koneko had who is the same age as her, Koneko is very close to Gasper and has been shown to enjoy teasing Gasper calling him "Gya-kun". During his training, she chased him around while holding garlic.
Xenovia, like Koneko, seems to enjoy teasing Gasper a lot, chasing him with her Durandal at hand. However, she, similar to Issei, trained him to become stronger- while the process rendered him to become scared of her during his first encounters with her.
Gasper's childhood friend and savior. Gasper cares a lot for Valerie, going berserk when he thought Valerie died. He vowed to return the stolen holy grail to her so she could wake up. Gasper was very happy when she did wake up in volume 19, shedding tears of joy. In volume 20, he is shown to be very protective of Valerie, getting upset when she went around the Hyoudoue residence on her own.