Blue Diamond

Blue Diamond is a character from Steven Universe and is a major supporting character in The Knights of the Multiverse, making her first onscreen appearance in Season 2. She's a member of the Great Diamond Authority that rules over the Gem race.

In Season 3, she and the other Diamonds would collaborate with the Guild of the Multiverse to bring Pink back home and destroy the Crystal Knights for, as they put it, corrupting Pink's mind.

Personality
Blue Diamond is a very mellow Diamond who treats high-ranking members of her court with respect, given her assigned role to act as an enforcer and arbiter of Gem society. According to Pink, she has a reputation for mercy and forgiveness (at least a lot more than Yellow). She is also extremely diplomatic, as she has a great respect for due process and an interest in the truth, sometimes even against Yellow's wishes. However, as with Yellow, she can often be extremely unforgiving towards some gems' more aggreges mistakes and is somewhat derogatory towards other races.

Yellow Diamond
Due to their roles as the Gem matriarchs, Blue and Yellow Diamond treat each other with respect, considering themselves equals. Their attitude toward legal matters suits their roles as overruling authority figures. As Yellow is a lot more militant, she considers trials as pointless and would prefer unhesitating action while Blue prefers to hear every opinion possible to determine the best possible decision even through slow, lengthy processes. Yellow's short-sighted and sometimes violent conclusions on  often anger Blue as well, often yelling at her for not restraining yourself. Despite their differing beliefs, they still care about each other, as when Yellow nearly broke down after singing "What's the Use of Feeling (Blue)?", Blue comforted her by placing a hand on her back.

Pink Diamond
Blue describes her past relationship with Pink as a very close one. Pink would often try to lift her spirits by doing things Blue described as "silly." However, Blue tended to disregard her opinions rather than acknowledge her as an equal in the Diamond Authority, sometimes even becoming frustrated that Pink tended to show little regard for Homeworld's strict societal norms, though she made an effort to bend the rules for her sake. She often treated her like a child, telling her to stop coming up with excuses and forbidding her from arguing back while at the same time reassuring her she doesn't have to worry about anything because of her status. When she couldn't overlook her transgressions though, Pink would often be locked in a tower and was often brought to tears by her sister's pathogenetic aura as further punishment.

When Pink went "missing", Blue was the one to lead the search for her, which soon led to the Diamonds discovering the existence of. When she did find her (and the Knights), though she was initially still trying to force Homeworld's way of life onto her, Pink's new "friends" and Pink herself managed to confront her and make her realize how abusive she was towards her whenever she went against Homeworld's ideals, even making up for it by helping the group get back home.