From both manga and anime, it was shown that Ada along with Oz were both very close to Oscar as to often play with him and hear him read stories as kids. They share a loving family relationship as uncle and niece. Ada and Oscar also kept in contact as they exchange letters to each other, it was also proven that Oscar deeply care for his niece resulting in him dragging others to the Lutwidge in able to reveal Ada's crush. Ada also seem to show deep care and affection for her uncle as she cried as the news of his death has reached her and also through her words, as she stated that Oscar to her beliefs must have smiled before he died ( which he did) proving the fact that Ada understood her uncle very well.
Xai is Ada's father. He doesn't seem to see her much more than he sees Oz, although he undeniably prefers her over him. Although it is revealed that Xai loves Ada--at least to a point--when he dies protecting her, in previous interactions he appears to be almost completely apathetic around her, willing to do anything to achieve his goal of hurting Oz. Ada's feelings on Xai aren't elaborated much. The only things than can be discerned or assumed are that Ada seems to dislike Xai because of how he has always viewed Oz, and her distaste for him grows when he kills Oscar. Ada also has taken Oscar and Oz's sides rather than Xai's on several occasions without hesitation. Despite the fact that Ada does not hate Xai (as it can be noted that Ada doesn't seem to truly hate anyone), it is interesting to note that after Xai's death, Ada does not appear to be grieving at all.
Gilbert and Ada have a very good relationship. Being Oz's servant, Gilbert used to spend much time with Ada as well. It was revealed in a Drama CD that Ada used to tell Gilbert that she would be his mistress when she grew up. After Oz disappeared into the Abyss, it is possible that Ada and Gilbert grew closer together to close the emptiness. Ada is the one who gave Gilbert his hat, which he treasures. Ada has the habit of blushing when she sees Gilbert's face, as he looks like Vincent whom Ada is in love with. Though it is not shown much in the manga or anime, it was revealed in the Caucus Novels that Gilbert considers Ada to be the ideal type of woman, thinking that she is warm, kind, gentle, and innocent, with no bad intentions whatsoever. He often wishes to see her when he is depressed, due to her sweetness cheering him up. He even tells Reim, "If only all women could be like Miss Ada...". Though he has shown no real interest in dating her, it is hinted that he has romantic feelings for her but is completely oblivious to them himself. Vincent, however, seems to realize this. The reason he started dating Ada in the first place was because he knew that Gilbert considered her to be "his most important woman". Vincent also constantly tells Gilbert "Brother, that girl is no good..." or "Yes, you should really give up, brother... on that woman..." due to Gilbert not knowing about Ada's obsession with the occult.
In the anime (not the manga) during the episode where Oz, Gil and Uncle Oscar are investigating Ada's school to find out who Ada's crush is. It is hinted that Ada shows signs of interest in Elliot, yet Elliot treats her badly due to their family's relationship having bad history together. But deep down, Elliot must have cared enough for her, because of the fact he bothered to search for her when he noticed she had gone missing when walking down the halls of the school when he saw her cats acting strange. Elliot Nightray had saved his upperclassmen and her brother from being held captive of the Baskervilles. With out thinking about it, he over passed his pride and carried Ada back when she was still 'asleep' from the poison. Question still remains on who gave her the antidote. Oz? Elliot? Or was it even Elliot's servant Leo?