Unlike Hannah and Barbara, Diana does not resent Akko for her not being of magic blood and does not flaunt her auspicious origins. Rather, she is frustrated by Akko's attitude towards magic, which she sees as naive and foolish. Diana believes that magic requires discipline and can only be acquired through assiduous research and an accumulation of traditions. Akko, on the other hand, feels that magic is something that comes from the heart and can be acquired through determination and steadfast belief in herself. Diana does not think Akko is serious about becoming a witch because of this attitude. For the most part Diana's view is correct, her magic is consistently exemplary while Akko's is inconsistent and often sloppy. However, Akko occasionally performs great feats of magic rivaling even Diana's through pure strength of will, visibly shaking Diana's worldview. (Pappiliodya and Samhain Magic) Thus, Diana gains some respect for Akko's "stubbornness".
She has also gained an interest in Akko's activities due to her studies of the Grand Triskellion since Akko possesses the Shiny Rod. While initially, she wanted to acquire the power for herself and use it to help people and restore magic in the world, as her mother intended her to do, in her studies she learned that only the person chosen by the Shiny Rod can awaken the words and use this power. She reveals all this to Akko in Episode 19 and explains that Akko is that chosen one. While she is understandably saddened by this and frustrated that the one destined to restore magic to its ancient glory is so ignorant of its history, she is not resentful. She has respect for Akko and defends her from the mocking of her family, saying she is as much of a witch as she is. She is still concerned with keeping up appearances because she takes the ultra-rare Shiny Chariot Card outside her room to prevent Akko from discovering it.
After Akko inspires her to become the Cavendish head of the household and aids her in the finishing ascension ritual, Akko and Diana's friendship gets greatly improved. Around this point, it can be argued that Diana begin to consider Akko as the only true friend that she ever had.
In "Yesterday", Diana shows a great concern for Akko, confronting her idol Chariot (Ursula Callistis) over the latter's seeming abandonment of Akko in her greatest time of need. Diana finally tells Akko the truth about her love of Shiny Chariot, and the two bond over their common passion, even as both recently had upsetting encounters with real-life Chariot. Diana gives Akko her precious premium Chariot card to remind Akko the truth of "A believing heart is your magic", which had inspired her to regain her own magic after it being absorbed by Chariot.
In "Tree of Leaves", Diana was shown to genuinely care about Akko. As Akko was knocked out of the Shiny Ballista by the Magitronic missile, Diana was in panic and even started sobbing until she saw that Akko was unharmed, relieving her tension. After magic is restored and the world has been saved, Diana is shown to jest about Akko's failure in broom flying, implying they have a close yet somewhat rivaling relationship now.
Hannah and Barbara are Diana's best friends, always flanking and flattering her. They are not mere sycophants, but rather they look up to her and trust her insight into magical matters unconditionally. In the short films, Diana would frequently lead the insults against Akko while Hannah and Barbara reinforced them. In the series, this is changed to while she accompanies them almost everywhere, she is far more mature than they are and doesn't find fun in tormenting Akko. While she is generally tolerant of their constant commentary, she gets annoyed when they flatter her excessively and occasionally cuts them off when they go too far in teasing Akko, as seen in "Samhain Magic".
As children, Diana met Andrew with whom he frequently exchanged pleasantries in his summer residences. Both used to have fights about whether magic really had any value at present. Despite this, they don't consider each other actual friends upon reuniting in their adolescence, especially due to their conflicting views on magic. However, their relationship throughout the series grows to that of mutual understandings, and arguments become more playful as they engage with each other on better terms.
Lotte respects Diana's skill with magic but defends Akko from her insults. Diana's feelings remain neutral towards her, although she was hinted to also care little about her in the short films.
Sucy's mind reveals she considers Diana very arrogant but nevertheless respects her magic abilities. Diana is neutral towards her, although she was hinted to also care little about her in the short films.
—Diana to Ursula, Yesterday
Like other teachers, Ursula recognizes Diana's skill in magic, however she has a particular respect for her hard work and dedication. From her research Diana realized that Ursula Callistis is in fact her (now secret) childhood idol, Shiny Chariot. Upon learning that Ursula accidentally absorbed Akko's (and unbeknownst to Ursula, also her) magical abilities, she is implied to forgive Chariot, but is furious that she did not pursue Akko when she ran off at the same realization. As she shown allow Ursula to assist her and her friends stopping Noir Rod that possesing missile and progressively reform its draconic form through it, it's plausible that she had let go of her bitterness towards Chariot.
Bernadette was Diana's mother who loved her daughter very much. Despite her poor health, she always spent time to talk to Diana the story of their ancestor Beatrix and their bonds together are the greatest drive on Diana's life.
Anna is the Cavendish family's maid and Diana's parental figure after the passing of Bernadette and her husband. She had served her family since Diana's childhood and did her best to support her.
The relationships between Diana and her father was not explored up to this point, but it's implied that she and her father were close as much as with his wife Bernadette. It's even implied that he was the man who seen sitting beside young Diana's when she attended Chariot's show.
Daryl is the current head of House of Cavendish and Diana's aunt. Diana's relationships with Daryl is strained at best due to their clashing opinion about importance of tradition and their family's legacy. This later took turn to the worst when Daryl shamelessly attempted to sell her own family's heirloom to Hanbriges, angering Diana who more than determined to take over the position of Head of House of Cavendish, in which Daryl responded by attempted to stop her niece twice, resulting her and her daughters exposed to curse that fall on anyone who attempt to disrupt the ritual to become Head of Cavendish. Fortunately, after Diana saved her and her daughters from the curse that fall on those who dare to interfere ascension ritual (and gained Beatrix's silent praise), Daryl finally sees her errors and realized that Diana's compassion and stubborness, a trait she shared with Bernadette, are what made her a better future Head of Cavendish.
Upon hearing the news about Diana and Akko's fight against Noir Missile, Daryl, and the rest of household of Cavendish family, are among those who gave them their Fuel Spirit to both restore magic to the world and destroying Noir Missile before it further evolve into apocalyptic monster.
Maril and Merril are Daryl's daughters and Diana's cousins. As they always support their mother in her greed and self-destructing habit, this put them into odds with Diana. After they and her mother saved by Diana from the curse that fall on those who dare to interfere ascension ritual, it's implied that their relationships improved to some degree, as they and the rest of household of Cavendish family, are among those who gave them their Fuel Spirit to both restore magic to the world and destroying Noir Missile before it further evolve into apocalyptic monster.
Diana respects Finnelan as she does all teachers. Finnelan has a great appreciation for Diana as a model student, reaching the point of favoritism.
Diana saves Thomas in The Enchanted Parade. Thomas gained a mild crush on Diana when she complemented him for catching the Shiny Rod after Akko accidentally dropped it.