WARNING: The following contains spoilers for New Mutants #15 by Vita Ayala, Rod Reis & VC's Travis Lanham
Exlir has taken a major position within the mutant eco-system in the X-Men's nation Krakoa, as his impressive healing abilities make him a vital part of the Five's resurrection capabilities. He's still a relatively young mutant, however, and it's worth remembering -- especially when it comes to how he interacts with other members of the X-Men.
In an attempt to get her son revived sooner than later, Wolfsbane just called on Exilir for help in New Mutants #15, calling back to the unethical teacher-student romance the pair shared when she was an instructor at the Xavier Institute.
Although many mutants have been reborn on Krakoa thanks to the actions of the Five, there are still some notable figures who haven't been reborn, including Tier, the son of Wolfsbane and the Asgardian Wolf Prince Hrimhari. Killed shortly after his birth by Strong Guy to briefly cement his control of Hell, Tier disappeared in a flash of light in 2013's X-Factor #256, by Peter David and Emanuela Lupacchino. It turns out that Wolfsbane has been trying to find a way to get her son back into the queue for resurrection, even calling upon a member of the Five personally for help. Elixir responds in the form of a letter, telling her he'll try to help but explaining the difficulties with proving Tier's death, which makes it that much more difficult to add him to the revival queue. The entire letter has a touch of affection, hinting that Exlir still has some form of feelings for his former instructor.
The two have a deep but problematic relationship that grew out of the time as teacher and student during Nunzio DeFilippis, Christina Weir and Carlo Barberi's New Mutants run in the 2000s. The pair first met when Elixir was a new student at the Xavier Institute, and Wolfsbane became an instructor at the Xavier Institute. At the time, she had been depowered during a fight with Mystique and had just lost her adopted mother, Moira MacTaggert, but Exlier's surprising advances ended up restoring her powers. Wolfsbane soon became a supervising staff member at the school (and the chief instructor of the Paragons Squad), but the elder New Mutant continued a secret romance with Elixir. Even though Wolfsbane has been a teenager since the early '80s, the sliding timescale of the Marvel Universe meant that they weren't far apart in age, but they both knew it was still wrong. Despite knowing the implications of such a romance, Wolfsbane couldn't stop herself from being drawn to the young man.
Despite this, Elixir also ended up catching the attention of fellow young mutant, Wallflower. Adter seeing Exlir go on a date with the young woman and even share a kiss, Wolfsbane tried to end their romance for her sake. Unfortunately, the pair were overheard by Wither -- a young mutant who'd grown incredibly jealous of Exlir in their time at the school. Seeing a chance to hurt him, Wither revealed the existence of the romance to Emma Frost. Confronting Wolfsbane, the young woman revealed the truth of the relationship. Dani Moonstar was furious with her friend, while Cyclops was at least able to understand how her mental state at the time could cause her to make such decisions. Wolfsbane ended up fleeing the school over the incident, eventually ending up as a member of Jamie Madrox's new X-Factor Investigations.
Despite there only being a three year age difference between the pair, Dani made it clear that the relationship was wrong because Wolfsbane has reached a point in her life where she was an adult in a position of authority/ Even if she was attracted to Exlir, she should have recognized he wasn't her peer, and it was, therefore, wrong to allow him to pursue her romantically. The fact that Wolfsbane was an instructor at the school at the same time makes it worse, as it meant she had a position of authority over him for their entire romance.
While both have moved on from their brief flirtation, it's notable that both of them still regard it as an important relationship in their lives, and it's apparently not one that Exilir completely regrets, either.