Tsubaki's attitude is more reserved and quiet which makes her the complete opposite of Black Star. However, despite their different personalities, they are rather close, sharing a strong relationship. Black Star still brags to Tsubaki about his abilities like he does with everyone else, and becomes impatient with her when she does not agree with him, up until the point he abandons her to fight on his own at one point. But despite these occasional setbacks, Black Star still genuinely cares for Tsubaki, and is very protective of her. As Tsubaki is the only one who had not ignored Black Star and the only one who had accepted him as he is, she means a lot to him. He knows that Tsubaki is a person that can be relied on, and also does not like to worry her. Although Black Star is not normally good at reading other people's emotions, he catches on easily to Tsubaki's, and always tries to cheer her up when she is upset, most notably when he offers her a hug after she defeats her brother. His arrogance is toned down a little when he is with Tsubaki, and he acts more relaxed, showing that he can be more like himself around her. Tsubaki is also one of the few people that Black Star actually listens to, and he occasionally even follows her advice at times.
While he does not express it directly, he appreciates Tsubaki fighting alongside him and helping him, and the two work well together in battle. He occasionally depends on her to sort out anything that he cannot, such as fights between him and his friends. Consequently, he is prepared to shoulder a great amount of responsibility in order to help her in any way. He decides to follow the 'Path of the Warrior' in order to fulfill Tsubaki's brother's unfulfilled dream, which results in Tsubaki being reduced to tears. Black Star also appreciates Tsubaki's support, and sees her as a light in the darkness of his heritage as a member of the Star Clan, and he comments that if he is the dark night sky, then Tsubaki is the shining stars. At one point, he also calls her by the affectionate term, 'Tsubaki-chan.'
Whilst in the Lust Chapter of the Book of Eibon, Black Star's appearance becomes a manifestation of what he finds attractive in the opposite sex. He greatly resembles Tsubaki in this form, which may suggest that he even has romantic feelings towards her. He does seem to find her physically attractive, as he proceeds to peep on her whilst she is bathing a couple of times. The two also live together in the same flat, and Black Star later confirms that they also sleep in the same room.