The Kingdom of Mistral and Ozpin originally put Leonardo in charge of Haven academy to improve relations with Faunus as Mistral was one of the most racist Kingdoms towards the species. In RWBY: The Grimm Campaign, it was revealed that due to being a Faunus and despite being a member of the council, Leonardo didn't have many friends in the Kingdom. While this was true, RWBY: Roman Holiday revealed that he was the most powerful person in the kingdom with only Lil' Miss Malachite coming close behind him.
It was shown in "When Life Gives You Lemons" that he obtained the distress signal Team SAFR sent from Kuchinashi and sent Huntsman reinforcements to help them retake the city from the Wave and chase off the Hana Guild.
Despite betraying the Kingdom to save himself, Ozpin decided to cover up his betrayal and make it seem like Lionheart died heroically due to wanting him to be remembered by Mistral for all the good he had done in his life instead of his betrayal. Following his death, Lil' Miss Malachite became the most powerful person in Mistral.
Lionheart was the Headmaster of 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. By the time Ruby Rose and her friends arrive at Haven, there were no students at the academy due to arriving in-between semesters and there were no academy staff there with the exception of professor Lionheart. Because of this, Qrow started to suspect something was wrong at Haven since there should have been more staff at the academy at that point besides Lionheart.
It was later revealed that he was reluctantly working with Salem to help her obtain the Relic housed in the Academy and used his position to help Salem eliminate Huntsmen and Huntresses sent out on missions along with the staff members of Haven Academy. Following Lionheart's own death, Haven Academy was shut down due to the deaths of all of its staff and the students were all transferred to Shade Academy as a result.
While Lionheart aided Salem's forces with attempting to destroy Haven, Ozpin later decided to cover up his betrayal by making it seem Lionheart died heroically defending Haven instead.
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". When Lionheart told her about the location of the current Spring Maiden and asked her to hurry, Salem showed anger at him and had her Seer strangle him into submission. In "True Colors", he confessed to Raven Branwen that he joined Salem because he had lost hope of her ever being defeated.
Later in "The More the Merrier", Lionheart believed that if he gave Salem Ozpin's current reincarnation Oscar Pine to her that she would let him go free from the conflict as a reward.
In "Haven's Fate", Leonardo told her that Cinder was to blame for the attack on Haven failing and tried to attack the Seer Salem was using to communicate with him after she learned Lionheart was planning on running away from the conflict, but was easily disarmed as it grabbed hold of him. After this, Lionheart begged Salem to spare his life and told her that he could still be useful, but she instead had the Seer kill him and called him a coward.
It's implied that Lionheart only started working with Salem during the two years between "RWBY: The Grimm Campaign" and the beginning of the main series due to learning about her immortality.
Arthur was the primary link between Lionheart and Salem. When Watts visited him in "No Safe Haven", Lionheart gave Watts a cup of tea from the same teaset that he got as a gift from Ozpin.
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 and watched watched him suffer as Salem used a Seer to strangle him with no concern for him. After Salem kills Lionheart, Watts was shown to not care about his death.
While Lionheart and Hazel worked together during the Battle of Haven, Hazel threatened Lionheart after he got knocked into him by Oscar Pine before Lionheart told him that he was Ozpin's current reincarnation. Following Lionheart's death, Hazel didn't seem to care about his death.
When Jaune Arc asked Lionheart about Cinder Fall actually being a student from Haven, Lionheart told him that she forged documents to fake being a student so she could infiltrate Beacon Academy without raising suspicions.
Cinder was later the one who revealed the full extent of Lionheart's betrayal, citing him as the reason she and her associates were able to enter the Vytal Festival due to him helping forge documents for them.
Despite fighting together during the Battle of Haven, Lionheart didn't actually like working with her. When Lionheart tried to escape from Haven and was caught by Salem, he told her that Cinder was the one to blame for their failure due to her changing the original plan.
Because of their jobs and their roles in keeping the Maidens safe, Lionheart and Ozpin were allies. In the past, Ozpin stated that he thought Lionheart was a great man and Huntsman and that he wouldn't turn against him. Lionheart also received a teaset from Ozpin as a gift for their friendship, something he kept in his office.
However, according to Qrow, Leonardo had not spoken to Ozpin in some time even before the Fall of Beacon. Later, Ozpin learned that Lionheart had betrayed him and became a cowardly man in the process.
In "True Colors", Lionheart confessed that he knew he has betrayed Ozpin and strongly felt that Ozpin shouldn't forgive him for what he had done to help Salem.
In "The More the Merrier", Lionheart was confronted by Ozpin's new incarnation, Oscar Pine as Ozpin wondered what happened to him. During their battle, Lionheart entertained the notion of delivering Ozpin to Salem, in hopes of being "free."
After Lionheart was killed by Salem following the Battle of Haven, Ozpin decided to cover up Lionheart's betrayal and make it seem like he died heroically defending Haven Academy despite thinking what he did was reprehensible.
It's implied that Lionheart betrayed Ozpin's cause due to learning about Salem's immortality and only started helping her during the two years between "RWBY: The Grimm Campaign" and the beginning of the main series.
When they first met in "The More the Merrier", Lionheart originally asked Oscar to leave unless he wanted to get hurt before he reluctantly fired at him. After he learned that he was Ozpin's current reincarnation however, he believed that if he gave him to Salem that she would let let him go "free" from the conflict and attempted to capture him.
After Oscar saw Ruby knocked unconscious, he hit Lionheart hard enough to fall from the balcony they were fighting on. Following Lionheart's death, he also saw Lionheart as a coward and a traitor for saving himself.
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 was revealed by Raven and Cinder in "The More the Merrier", Qrow was disgusted by the Haven Headmaster's actions and later engaged him in battle. Following Lionheart's death, Qrow lost any remaining respect for him.
As shown by "The Mission", Qrow had a low opinion of Leonardo's competence and ability to control Mistral even years before the Battle of Beacon, disregarding any effectiveness due to Lionheart's Faunus trait.
Because of their roles in guarding the Maidens, Ironwood 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, Lionheart disapproved of Ironwood's recent actions like his Dust Embargo, and how his personality had changed after the Battle of Beacon.
After Ruby's Group revealed to Ironwood that Lionheart helped Salem even before the Fall of Beacon out of fear of her, he swore to Oscar Pine in the Atlas Vault that he wouldn't end up the same way. However, while Ironwood didn't turn like Lionheart, he instead went in the opposite direction and intended on sacrificing anything to stop her.
The two were seen together in Lionheart's office in "True Colors" after he ended his call with Qrow. Lionheart was ashamed of what he had 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.
Later in "The More the Merrier", Raven defended Leo for betraying Ozpin by stating it was what any "sane" person would do. While they fought together during the Battle of Haven, Lionheart never learned that Raven was the real Spring Maiden before his death.
Lionheart and Vernal didn't interact with each othet. 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 revealed he knew where she was, but he was then saddened to learn Vernal had fallen in with the Branwen Tribe. After he learned about this, he told Salem of Vernal's location.
Lionheart never learned that Vernal was just a decoy for Raven and that she was killed before his own death.
- Volume 5 Directors' Commentary Chapter 1