Arthur Cummings Jr. is a 17-year-old member of the Cosmic Culture Club, an international exchange program visiting the United Nations Mars base when alien forces attack. He survives the initial assault alongside five other club members and their instructor, Elizabeth Clabery, and is tied as the oldest member of this survivor group.

Generally well-liked within the Cosmic Culture Club, Arthur maintains a close friendship with the younger, intellectually gifted Roanne Demitrich, often serving as her confidant. However, he displays tendencies toward emotional weakness, cowardice, and a fragile will, frequently manifesting as emotional outbursts, ramblings, and difficulty maintaining composure under pressure.

A significant aspect of his personality involves his deep romantic feelings for fellow survivor Simone Reflann, making him particularly susceptible to her influence. Simone exploits his eager-to-please nature, using his attraction to manipulate him into complying with her requests or participating in activities he might otherwise avoid.

Throughout the conflict against the alien Gradosian invasion, Arthur remains primarily within the group of surviving youths. Unlike other members who take on active combat or strategic roles, such as David Rutherford and Roanne Demitrich becoming pilots supporting the protagonist Eiji, Arthur's role within the resistance effort is less defined. His personality traits remain consistent, with no specific information detailing significant character development or a transition into a more proactive role across the series, its supplementary OVAs, or the alternative ending presented in "Kokuin 2000".