Natsuki Subaru
Satella is madly in love with Subaru and claims to love him for "giving her light, showing her the outside world, holding her hand when she was lonely, and kissing her when she was all alone" essentially giving her everything and Natsuki Subaru does know what Satella said. She states that she gave him Return by Death to kill her, however he refused to do so, telling her that he will save her instead. Both personalities love Subaru, though the Witch of Envy personality is much more aggressive about it, while Satella is much more reasonable and cares deeply about his well-being.
As already mentioned, The Witch of Envy stands behind Subaru's Return by Death. Whenever Subaru breaks "their promise", Satella preforms some type of punishment, either by crushing his heart or (in extremely rare occasions), crushing the hearts of people he cares about the most. In arc 6, Satella repeatedly caused the destruction of the Pleiades Watchtower and killed everyone in it, as long as Rui Arneb was latched onto Subaru.
Emilia
Certain people, such as Echidna and Pandora, have referred to Emilia as "the witch's daughter", with Pandora also mentioning that she was of the witch's bloodline after seeing her power, hinting at some sort of connection between the two, though nothing has been explained, yet. According to Melakuera, Emilia was "born from the same cursed blood as the Witch" as well as calling her "the descendant of the Witch".
Abnormally, if Subaru tells Emilia about Return By Death, the Witch of Envy crushes Emilia's heart with one of her shadow hands and resulting her demise, as seen in one of the dead end loops of Arc 3. Other than Subaru, Emilia is the only person affected by this penalty of Return By Death, with no exact explanation to why this is.
Witches of Sin
It's worth noting that Minerva, Sekhmet, Typhon, Daphne, and Carmilla hate the Witch of Envy, not Satella. Echidna is the only one who hates her regardless but she can't do anything as the other Witches would join forces against her.
Of the group, Satella was closest to Typhon and Minerva.
Volcanica
Not much is known about their relationship, but Volcanica initially mistook Emilia for Satella and treated her with kindness. Volcanica seemed to be on friendly terms with Satella and showed some form of respect for her like an old friend. However, when Emilia mentioned the "Witch of Envy", Volcanica became aggressive and believed that Satella had been fully reduced to the "Witch of Envy". Volcanica also laments about his past when he talks to Emilia, believing that things would have turned out differently if he hadn't faltered on "that day".
Al
It is heavily implied that Al knows Satella personally. In Sunlight on the Water, Al encounters an illusion of a shadowy figure wearing a shadowy dress, which Al was unable to bring himself to harm even though he tried some 100,000,000 million times. Unable to do anything, he was gripped with fear, and tried to retreat as it attempted to touch him lovingly with its fingers. Fortunately, Priscilla destroyed it with her Yang Sword, yet this caused him to be filled with rage, though she easily dealt with him by smacking his helmet with her sword. Although he was still scared somewhat, he was relieved that the shadowy figure was fake.
In The Sword Wolf of the Scintillating Flame, Al states that at some point in his life, a beautiful silver haired girl appeared before him when he was about to die. He also states that silver hair brings him bad memories about how a "trash, stupid useless person he is" due to how he failed to save the one he cherishes the most.
Alec Hoshin
Currently, Hoshin and Satella's exact relationship is unknown, however, Hoshin states that he would have died in the middle of nowhere if not found by Satella in the past. He describes Satella as earnest and according to him, she would be searching for him with all she could.