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Ronan is a side character who made his cameo in Love (Part 2 to be specific). He is also a member of The HSO Military Defenses and is capable of using magic.
Ronan wears a grey-green hat and a long-sleeved grey uniform that has yellow triangles at the bottom of the sleeves. He has a badge located on his chest, perhaps displaying his employment under the Anti- Monster Department. He has brown hair, pale skin, and yellow eyes, which correspond with his Justice SOUL.
Because of his justice soul, he shows a personality that reflects his trait. Thus, he is fair and equal to everyone. Besides this, Ronan seems to carry a caring personality, as he asked Jessica Grey if she was alright. Despite him allying with the Anti- Monster Department (AMD), he seems perfectly willing to cooperate with the monsters in fighting against the Pink Blobs, meaning he does not fit the stereotypical, xenophobic profile of someone who would work with the AMD. He is brave and compassionate for those more vulnerable than him, and this is shown when he fights off a Pink Blob that is about to attack a girl.
Jessica Grey asked Ronan to give her the status report around his progress. Ronan then replied, saying the HSO military forces were set at all possible entrances and how they set up a forcefield, run by the town's best wizards and wizardesses, to prevent Betty and her minions from going anywhere else other than the town. Jessica mentioned Betty was able to absorb both monster souls and human souls. This is because Betty isn't a monster or a human. Rather, she is a spell. Jessica then warned Ronan, saying even if he let his men fight Betty or her minions, there may still be casualties because no one knows how to fight her or her minions. The conversation ended when Ronan said he would call Jessica if and when the AMD spies anything suspicious. Ronan also told Jessica to call him if and when she wanted to let his men in as well. Meanwhile, unbeknownst to Ronan or any other AMD members, she signed the peace treaty during the phone call.
Just after Jessica weakened her SOUL with the AMR and was stabbed by a pink blob, Ronan tried to call Jessica, but Asgore answered instead. Once Asgore tells Ronan that Jessica was injured and that "she might not get better (alluding to the damage in her SOUL, not her body)", he quickly became concerned despite Asgore reassuring Ronan that Toriel and he were trying to heal Jessica as much as they could. Ronan asked Asgore what had happened to Jessica. Asgore told Ronan that Jessica got attacked by a pink blob. Asgore asked Ronan to gather everyone to a safe area inside the city and to focus the HSO's defenses there. Ronan told Asgore that he will set a perimeter around Toriel's school and the arena and asked Asgore if that idea was okay. Asgore agreed to this, but suddenly got interrupted by Toriel, who got angry and didn't want people to gather at the school as it may increase the likelihood of the school getting attacked again. Asgore ignored Toriel and told Ronan to proceed with his plan. Ronan was seen again in the episode saving an unnamed girl from a pink blob. Ronan disintegrates the pink blob that was about to attack the girl using his magic, then ordered her to go to the school in order to reach safety. Ronan then decided to take care of the pink blobs himself, so he summoned a wall of magical attacks similar to the one Undyne spawned in Love Part 1. Then, he told the pink blobs that he wouldn't let them destroy the city.
Ronan may be a part of the higher position of the HSO military forces because a Justice soul is usually seen in charge of military and political issues. Refer to here. Evidence also shows that he is also in charge of other members as Jessica stated, "Even if you let your men in."