Gasper Vladi is a half-vampire male cross-dresser who has the ability to stop the time of those he sees but he is unable to control it. He is sealed by Rias as ordered by the elders till she has grown up enough to control him. He still is unable to have a full command on his powers but can improve a lot by drinking Issei's blood.
Originally a coward and a shut-in, Gasper likes to wear female clothing, claiming that girls' clothes are cute. He also has a penchant for boxes, carrying one with him all the time. When Issei tried to give Gasper confidence, the former gave Gasper a paper bag to put over the latter's face. However, Issei comments that whenever Gasper puts the paper bag on, the latter looks like a molester. As the series progresses, Gasper starts to get more confident and becomes braver.
Gasper first appears in Volume 4, after Rias was given permission to unseal him (much to his dismay). He then trains with Issei, Xenovia, and Koneko on how to control his Sacred Gear and with Azazel's advice and Saji's help, improve considerably. During the attack by Khaos Brigade on the leaders of the Three Factions, Gasper was captured and used against his will before being saved by Issei and Rias with Issei using his blood to allow Gasper to control his power as the two of them overpower the female Magicians.
Reason For Cross-Dressing
Thinks that girl's clothes are cute. In a Light Novel side story he temporarily becomes an actual girl thanks to azazels gender change ray gun - though his appearance doesn't change other than losing "Something Important".