When Tara and Willow meet, their proficiency at magic is about the same, but Tara's knowledge of the craft far exceeds Willow's. Tara reveals that she has been practicing magic for most of her life, as her deceased mother had also been a powerful witch. Throughout season 4, Tara acts as a partner and guide in Willow's witchcraft, teaching Willow spells and performing magic together. Willow, however, is inherently talented, despite being new to the craft, and begins to progress much faster than Tara in the fifth season, including experimentations in dark magic. Tara struggles with understanding her place among the Scoobies with Buffy (Sarah Michelle Gellar), the leader, with which has a very friendly relationship; Xander Harris (Nicholas Brendon), Willow's friend since childhood; and Rupert Giles (Anthony Stewart Head), their mentor. Tara's primary role throughout the series is that of Willow's partner. She feels somewhat useless until the fifth season episode "Family" when the entire Scooby Gang (Spike included) makes it clear that she is unquestionably a part of them. The episode introduces some of Tara's blood family: a cold, authoritarian father who has lied to her all her life (telling her that her magical powers are a result of her being part-demon on her mother's side); an overbearing brother; and a judgmental, repressed and repressive cousin; all of whom Tara dismisses at the end of the episode. Tara later reveals in "The Body" that her mother had died when she was 17. In the following episode, when Dawn is acting out, Tara confides to Buffy that she had to deal with her brother after her mother's death. No more of her backstory is revealed in the series.  As Willow's character grows more self-assured and powerful through the seasons, Tara takes over some of the role of being placed in peril and needing to be rescued. The fifth season's primary villain, or Big Bad, is Glory, a goddess too powerful for Buffy to fight alone. Glory tortures Tara in order to gain information on the Key, but Tara asserts willpower and resists Glory, resulting in Glory stealing her sanity, and prompting Willow to go searching for retribution. Glory states she feels "buzzed" after feeding on Tara's mind, indicating Tara's powerful intellect (and possibly suggesting her power as a witch).  Buffy scholar Ian Shuttleworth writes that Benson was able to "admirably" portray the same range of emotions inherent in Tara although the character loses her identity. Willow's powers are significant enough that she is able to battle Glory more effectively than Buffy, if not completely successfully. In the final episode of season 5, Willow uses her magic to restore Tara's sanity, significantly weakening Glory in the process.  Tara also becomes a guide of sorts, and a maternal figure. She appears to Buffy in a dream in the fourth season finale "Restless" to tell her about the arrival of Buffy's sister Dawn (Michelle Trachtenberg) and act as a translator for the voiceless First Slayer. Following the death of Joyce Summers and Buffy's sacrifice to save the world at the end of the fifth season, Tara and Willow move into the Summers house, taking Joyce's bedroom and becoming Dawn's surrogate parents.

Answer this question "Do they form a relationship?" by extracting the answer from the text above.
Tara's primary role throughout the series is that of Willow's partner.