Michael Guerin is in many ways the most complex character on "Roswell," and the most driven as well. Unlike Max and Isabel, he has grown up in a very unhappy environment, being bounced around from various foster homes with neglectful caretakers who have given him no support or encouragement. He lived in a trailer park with his latest foster dad, Hank, who was a mean and abusive drunk. As a result, Michael has become a complete outsider, coming close to flunking out of school despite his hidden brilliance, and trusting no one but Max and Isabel. He has become obsessed with finding out the truth about his origins as a means to escape the world that he has come to despise, and he often acts in an impetuous way to achieve this goal. Michael is also the most emotional of the alien trio, and the one most likely to act on his feelings. Michael has the least amount of control of his powers, due to his insecurity and inability to control his emotions.

His feelings for Maria threaten to disrupt his carefully maintained alienation from everyone except Max and Isabel. These feelings for Maria scare him a great deal, and he simply doesn’t know how to deal with them. Because of Michael's unhappy and uncertain childhood, he has not learned many of the niceties of human interpersonal behavior and also has built a shell around himself to protect him from being hurt. He is totally unskilled in relationships and his feelings for Maria are disturbing and frightening to him, so he wants to keep her at arms' length.

However, Maria is a very warm and accepting and loving person and, in spite of himself, they grow very close, especially after his mean foster dad Hank threw Michael out and threatened him with a gun (in "Independence Day"). Michael ended up crying in Maria's arms, coming to her as the only person who could comfort him. Hank disappeared after that and Michael was granted legal independence (Hank was killed by Nasedo, but Michael was unaware of this). Michael got a job as a cook at the Crashdown Cafe and an apartment and began to build a life on his own. He and Maria continued to grow even closer.

However, events soon conspire to push Maria and Michael apart. Tess arrives in town and Michael starts getting visions that he might be with Isabel - at one point they think she might even be pregnant with his child through some alien dream connection. Michael thinks for a time that Riverdog (Ned Romero) is Nasedo, and that Nasedo might be his father. When he meets the real Nasedo, it is quite a let down - nothing is as he imagined. Michael uses his powers to kill Agent Pierce, then feels he has become a bad person and a danger to Maria. He pushed her away totally - because he loves her and wants to protect her from him, and also because of the mission he has to do for his alien people.

The second season finds Michael with new, non-spiky, longer hair and a motorbike. He is still not the ideal boyfriend or the most charming or tactful person. If anything, his anger and frustration have increased and he is annoyed by Max's sudden appointment as king and leader, and they argue frequently.

Michael comes to trust Sheriff Valenti, and with his mission and finding out more about himself, he has overcome some of his anger and found new strength and new focus. Together the pod squad discovers new facts about their alien and human origins.

Michael and Isabel have rejected any thought of a relationship with each other. Maria and Michael continue to bicker and be close, but he initally stands firm on his resolve not to continue his romantic relationship with her.

The battle with the Skins heats up and Michael is not sure he is ready, or that he will be strong enough. The new waitress at the Crashdown, Courtney, comes on to him outrageously, amusing Michael and making Maria very jealous and annoyed. Michael is not interested in Courtney at all, but they become closer when he finds out she is an alien - a 'good' Skin and on his side. In a very touching gesture, in the episode "Harvest" Michael saves Courtney's 'husk' (her new human body being grown to replace her current shedding old one) before they destroy the Skins' headquarters in Arizona. Unfortunately, she dies to protect them soon afterward to keep the secret of the Granilith from falling into the hands of Nicholas, leader of the 'bad' Skins.

Michael has the unsettling experience of meeting his 'dupe' Rath, who is ten times more uncouth than Michael ever could be. The totally unbelieving and cynical Michael discovers some faith during Christmas when he helps Max heal sick children and Isabel helps him out and manages to get Maria a Christmas gift she loves. Also during the holiday season, there seems to be the beginning of a thaw in Michael's resolve to keep Maria at arms' length.

As Max grows more confident and stronger as a leader, Michael begins to doubt Max less and to become more confident in his own powers as well. He wants to forge ahead with things and still chafes under Max's cautious wait-and-see approach to life; while Max sees Michael as a person who 'leaps before he looks.' But Max never doubts Michael's friendship or loyalty, or that Michael will be there for him and will continue to be the driving force in their search for their identity and their origins.