During his early childhood, Rean was raised in Heimdallr, where he lived peacefully with his parents. This all changed in S. 1192 when a group of nobles targeted Osborne in an attempt to suppress a commoner-led reformist movement in which he was a prominent leader. During a jaeger raid on his home, Katia was killed and Rean was critically wounded by wooden debris to his chest. In order to save his son's life, Osborne transplanted his own heart into Rean.
Several months later, Osborne left Rean outside Ymir, where he was later picked up by Baron Teo Schwarzer. Schwarzer took Rean into his household and raised the boy with his wife Lucia and their three year old daughter, Elise. Since Rean had no memory prior to his abandonment, he did not know whether he was born a noble, commoner, or even an Erebonian to begin with. This sparked some controversy among the Baron's fellow nobility and many rumors spread about Rean being his bastard son. Nevertheless, the Schwarzers remained a kind and supportive family throughout Rean's upbringing.
Kind, sociable, easygoing and a modest to a fault, Rean is about as upright as they come. He is incredibly supportive, often helping people (friends or otherwise) with their various problems, often going through extreme lengths to fulfill requests. He is a natural born leader, often taking the initiative when the group faces a problem, relying on his good instincts and deductive reasoning skills to get through them. However, despite putting on a confident front, Rean has shockingly low self worth. He tends to give out harsh criticism of himself, such as when described himself as a "dropout who's never progressed past beginner level" when prompted about his sword style. He is also unable to reconcile the love that his family has for him, often considering himself a to be a burden, striving to one day "pay back his parents" for caring for them, unable to realize that his parents love him unconditionally.
He also has a tendency to put others over himself, often to his detriment. Jusis Albarea points out that his tendency to do this comes across as more abnormal than virtuous. He holds himself back when fighting due to the fear of losing control and accidentally hurting someone with his power. He is able to slowly overcome his flaws over the course of both games, eventually learning to accept the love that people have for him and accept his unknown power as an extension of himself. He deeply cares for his sister and is visibly annoyed when Patrick expresses interest in her and has gone on record that he won't just let any man be with her.
Rean is surprisingly competitive, often putting his all into winning competitions. He admonishes Group B for their abysmal grade in their previous field study, and spurs them into cooperating by appealing to their sense of pride. He will express dismay when losing to his opponent during minigames.
Rean has a fondness for fishing and enjoys snowboarding; he even helped construct Ymir's snowboarding course as a child. If the Unspeakable R costume set is anything to go by, Rean was quite imaginative as a child.