Euclid Lucifuge/Relationships

Rizevim Livan Lucifer
Rizevim has great trust in Euclid as he considers him to be his right hand man.

Grayfia Lucifuge
His older sister who he admired when he was younger, however that admiration was shattered when she rebelled against the original Lucifer and fell in love with Sirzechs. Despite baring a grudge against her, Euclid appears to still have some feelings for her, going as far as to kidnap Rossweisse due to her slight resemblance to his sister.

Sirzechs Lucifer
Euclid appears to have a personal grudge against Sirzechs for having his sister Grayfia's affection, even so he still refers to him as "brother".

Issei Hyoudou
Issei and Euclid's relationship started immediately hostile, as the latter wads resposible for the abduction of the first year members of the Occult Research Club and sending several Magicians to attack them and even aiding the revival of the Evil Dragons.

Furthermore, later on, both Issei and Ddraig developed a personal grudge against Euclid for creating replica of the Boosted Gear from the remains of Issei's previous body left in the Dimensional Gap and sacrificing the souls of many famous Dragons.

During thier fight in Volume 16, Euclid was able to overpower Issei's Cardinal Crimson Promotion with his own Scail Mail armor in it's normal Balance Breaker form, this states that Euclid's natural strength is equivalent to a Maou's even without the armor. However Issei was able to turn the tables on Euclid only after using his newly awakened power and further theories that Euclid was observing him in order to learn how to use more of his new techniques.

Rossweisse
Euclid took an intrest in Rossweisse in Volume 17, for her magical abilities and most of all on how she resembles his sister Grayfia, going as far as to kidnapping her which ultimatly failed.