Harley's relationship with Fang isn't easily described. She tries to act sort of like an older sibling to most of the younger people of the group (Fang, Tiara, Galdo) and often gives them advice, but she seems to tease Fang somewhat, albeit whether or not this is deliberate or do her lazy nature are unknown. She is more interested in his Fairy, Eryn and constantly nags Fang over swapping with her. Overall Harley recognizes Fang's strengths as an individual and leader to the group due to his charismatic nature.
Harley's relationship with Sherman is unlike the other females. She doesn't seem smitten with him, and is more interested with her research than him. The two seem distant, probably due to their personality's conflicting. She comes to trust Sherman as an ally before the singularity, but afterwards like everyone else views him as a negative and evil person.
Initially Harley was interested in Eryn because she was "Cute" but she also takes on an elder-sister role, more so for Eryn than any other of the Fairies. In a private scene between Eryn and Harley, Eryn confides to Harley about possibly being in love with Fang, and Harley reassures her that time will tell, and that she has nothing to worry about as long as she follows her heart. However, she still constantly tries to "Examine" her from time to time.
After the singularity, their relationship remains the same. She takes a renewed interest in her after Fang's fury evolves.
The two are like big sister and little sister, with Harley offering advice and concern towards Tiara, and Tiara likewise.
While their relationship seems to be mundane in the Evil Goddess Route, Marianna forbids Harley to do any research on her fairy Khalara, as he was terrified by her examination.