MARI is a major supporting character in OMORI. She is the older sister of OMORI in HEADSPACE, and SUNNY's older sister in the REAL WORLD. Her presence as a whole also serves as one of the major driving forces behind the game's story. She's often spoken of as being kind and gentle, and plays a sisterly role in the friend group with SUNNY, HERO, KEL, AUBREY, and BASIL. Her first appearance shows she has a mischievous side, poking fun at KEL and AUBREY's mannerisms and afterwards teasing HERO. These traits are shared with her HEADSPACE counterpart.

During his youth, HERO is the peacemaker of the group, who tries to restrain his friends whenever they get too worked up. He is described as having a mild personality and passionate in domestic work like baking, cooking, and cleaning, making him an ideal employee. HERO also has a charismatic side that he uses to charm others. His charisma allows him to convince others to go along with his requests, get discounts from MAILBOX and befriend animals like EMS. However, he can sometimes be a pushover when confronted, and has difficulty refusing others. He also appears to have arachnophobia (an irrational fear of spiders) to the point of freezing up completely. These traits are shared with his HEADSPACE counterpart.

BASIL is depicted as a relatively shy boy, well seen right after being introduced to the friend group, being rather insecure and fearing the thought of being perceived as a burden. He enjoys taking pictures, as his memories with his friends are his most treasured belonging, perceiving them as a way to permanently capture the moment. He also enjoys taking care of flowers, art, and reading. BASIL seems to develop strong emotional attachments to many of his possessions, as he will never throw away anything, no matter how worthless it is.

During her youth, AUBREY is very cheerful and always happy to play with her friends. Strong-willed and always acting true to her feelings, AUBREY serves as a constant source of morale for her friends. This extends to often reminding the group of the right thing to do whenever they are distracted. Despite this, she becomes sad and worried quite easy; she is the most anxious one of the friend group when BASIL goes missing. Often she is clumsy, resulting in yelling from KEL, who she always fights with. Her childhood traits are shared with her HEADSPACE counterpart, albeit slightly different with her puppy love persona towards OMORI.

During his youth, KEL is a bit bratty, impulsive and rude, always getting into little tiffs with his friends. He is very competitive by nature, always wanting to win games or races, and is indeed very energetic and fast. He also thinks romance is gross and that girls have cooties. However, even though he is loud and likes to annoy his friends, he genuinely cares about them to the point of constantly protecting them with a strong sense of justice. KEL appears to share a strong bond with his brother HERO and occasionally bickers with AUBREY over petty disagreements over minor topics. He even sees the quiet SUNNY as someone who is a great person and believes the best in him. These traits are shared with his HEADSPACE counterpart.