Lionheart was the headmaster of the Haven Academy. After The Fall of Beacon, many more Grimm had surfaced in Mistral, so Lionheart ostensibly had to shut his school down temporarily to send his students and staff to defend where needed. It is later revealed that he was reluctantly working with Salem to help her obtain the relic housed in the Academy and using his position to help Salem eliminate huntsmen and huntresses sent out on missions.
Lionheart was a reluctant ally of Salem and her associates, acting as a mole to the Mistral Council. He was a key to her plans.
Lionheart was Salem's informant in Mistral. Lionheart was very timid in her presence and was shown to be helping her out of fear, as seen in "Dread in the Air". In "True Colors", he confessed to Raven Branwen that he joined Salem because he had lost hope of her ever being defeated.
Arthur was the primary link between Lionheart and Salem. Arthur had a direct communication line between himself and Lionheart's desk. Watts also had little respect for the Professor, saying he should learn to improvise better.
Because of their jobs and their roles in keeping the Maidens safe, Lionheart and Ozpin were allies. However, according to Qrow, Leonardo had not spoken to Ozpin in some time even before the Fall of Beacon.
In "True Colors", Lionheart confessed that he knows he has betrayed Ozpin and strongly felt that Ozpin should not forgive him for what he has done to help Salem.
In "The More the Merrier", Lionheart was confronted by Ozpin's new incarnation, Oscar Pine. During their battle, Lionheart entertained the notion of delivering Ozpin to Salem, in hopes of being "free."
Qrow was an ally to Lionheart. However, Lionheart and Qrow disagreed on many things, most recently on how to deal with the missing Spring Maiden. According to Lionheart, he could not arrange a team to hunt down Raven and her bandits because the Council was busy with the Grimm hordes. However this may have been a lie on Salem's behalf. Leonardo was also less than thrilled to learn that Qrow partially filled Team RNJR up to speed on the situation.
Later, Lionheart contacted Qrow via Scroll to inform him of his "breakthrough" with the Mistral Council, knowing full well it was actually a trap set for Qrow and his allies. When the full extent of his betrayal is revealed by Raven and Cinder in "The More the Merrier", Qrow was disgusted by the Haven Headmaster's actions.
Because of their roles in guarding the Maidens, James and Leonardo were allies, but not on the best of terms. Ironwood openly mentioned he had no faith in Lionheart's abilities to handle things. Likewise, Leonardo showed open disappointment in the personality change Ironwood showed after the Fall of Beacon, specifically an increase in his more militant and controlling tendencies due to the General's closing of borders and Dust embargo.
The two were seen together in Lionheart's office in "True Colors" after he ended his call with Qrow. Lionheart was ashamed in what he has done, but Raven countered there was no shame in wanting to survive, regardless of the cost. Though when Lionheart asked who she was trying to convince, Raven promptly excused herself.
Lionheart and Vernal did not interact. Leonardo believed the ruse Raven Branwen had created: that Vernal was the previous Spring Maiden who abandoned her duties and fled Mistral over ten years ago. At first he was overjoyed when Qrow reveals he knows where she is, but he is then saddened to learn Vernal has fallen in with the Branwen Tribe.