Basteya Irclu is a sixth progenitor vampire who works closely with Saitō and the Hyakuya Sect. For the former it is serving out of practicality, Basteya at least entertained the idea of having their leash to Saitō cut. They profess to not have an opinion of humans despite working with them and attacking enemy ones. Basteya can work professionally with others regardless of who it is and their own personal views where the common tie is whether the allegiance suits the goals at hand. Basteya interacts with various other characters throughout the Seraph of the End series, which affects themselves and the story. This article details Basteya's relationships with other characters.
Japanese Imperial Demon Army
Shinya Hīragi
Having made their way to the top of Shibuya's walls, Shinya is the first one Basteya attacks. They grin when Shinya looks away seeing a second chance to assail him and happily asks whether he has time to talk before raising their sword with Shinya unarmed after being overpowered. Basteya left Shinya alone after they began speaking with Crowley.
Hyakuya Sect
Saitō
Lord Rigr Stafford is their leader and Basteya is loyal to him even addressing him as 'lord'. They do at least contemplate having the leash to him cut only dismissing the notion as the one who suggested it is a lower rank than them. Basteya shares with Ferid that it was Saitō who set his plan in motion concerning Shibuya and they simply play their part. Later Basteya shows the full extent of their loyalty to Saitō by allowing him to siphon their own power and life force to empower the Second Progenitor to battle on equal grounds against the First.
Hyakuya Sect Member
Vampires and humans are not traditional allies yet they are both members of the Hyakuya Sect so work together. Basteya does not react to this man's sudden appearance through a portal behind him atop of Shibuya's walls but makes use of it to withdraw from the battle.
Vampires
Crowley Eusford
Basteya would refrain from attacking Shinya at having his attention drawn to this vampire noble after his sword was clanged by Crowley's. They are familiar with him but does not give a proper answer on why they are here before suggesting that Crowley was on an errand or another for Ferid. Whilst Basteya is under the impression that Crowley is only a thirteenth progenitor and asks what he can do it may be under the context of how they doubt Crowley can realistically help them with his rank being significantly lower.
Ferid Bathory
Indifferent to Ferid's surprise attack on them, blocking it easily, Basteya does not take Ferid seriously as he was discarded by Saitō. When asked if Ferid should cut their leash Basteya notes that he cannot as they are a sixth progenitor while Ferid is seventh progenitor. Though Basteya asked what business it was of Ferid's at first when asked if Saitō is here, Basteya eventually shares with him that Saitō is indeed the one who set the plan concerning Shibuya in motion. Despite looking down on him due to his history with Saitō and even being attacked by him Basteya does not bear any overt hostility to Ferid.
Humans
Basteya declares that they do not have enough interest in humans to form an opinion of them either way when it is proposed that they could be friends with them, even when they have a human ally of their own. With Sayuri Hanayori and Shigure Yukimi with Norito Goshi and Mito Jūjō nearby Basteya was indifferent to Guren's squad approaching when they were speaking with Crowley.
Other
Shikama Doji
Basteya breaks into the room with an accomplice where Shikama Doji is in who is possessing Shinoa Hīragi. They work with Saitō in order to give him the capabilities to restrain and capture the first progenitor.