|“||Just a few seconds. That's all I need. Just a few seconds. If I can only stop Murakami...||”|
Yūko is a slender, curvacious girl with messy, wavy chin-length black hair and narrow green eyes. When not on active duty she appears to favor casual, comfortable clothes, such as a long-sleeved, dark-colored shirts which let her thin necklace and occasionally her shoulder straps show. She also appears to favor jeans. Her uniform consists in a futuristic-looking white body-fit top with a square collar, purple gloves, an indigo belt that wraps partially around her waist, and a shoulder strap exposed on her right side. The uniform sports six fake buttons arranged in two columns of three and a seventh right underneath the collar, on the right side; on the left is Border's emblem. The attire is completed by black pants that connect into knee-high purple boots.
One of Kumagai's defining traits is her determination. Despite being aware of the gap in strength between herself and Kō Murakami, she resolved to face him nonetheless, steeling herself so as to check her doubts, and smiled as she remembered her captain's incitement right before the fight started. She is also persevering, giving the fight against Murakami and Yūma Kuga her all and making the first move against the former when the latter left, another instance of her fearless pragmaticism against adversity. Her tears after her defeat were due at least in part to the deep bond she shares with her teammates and their vow to reach the top tier of B-rank before the end of the season. It is worth noting that Kumagai never displayed any animosity against her opponents, not even the Neighbor Wen Sō.
Nasu and Kumagai share a strong friendship and care deeply for one another, so much so that Nasu was outraged by Wen Sō disguised herself as Kumagai and, right before defeating the Neighbor, she claimed that she was taking revenge for the injuries she had inflicted on her friend. She calls Kumagai by her nickname, while the latter calls her by her first name. In combat Kumagai often serves as Nasu's bodyguard.
Hiura was scouted by Kumagai,[BBF Q127] who is classmates with her brother. Kumagai is somewhat of a mentor figure to her and directs her in combat. As with the other members of the unit, however, their relationship also extends beyond that into friendship, with Kumagai being genuinely saddened at the prospect of Hiura leaving the team. Hiura addresses Kumagai by her surname, while Kumagai uses her first name.
Kumagai is the one who scouted Shiki and brought her on as the unit's Operator. The two seem to be the most active while brainstorming the unit's strategies. They appear to be close, as Kumagai addresses Shiki by her first name.
Jin has a history of catching Kumagai by surprise and smaking her backside without her consent, which she occasionally reacts to by punching him in the face and warning that she will report him if he keeps it up. However, she does not appear to genuinely resent him, as she immediately dropped the discussion when Jin warned her that the aces her unit would fight against in the next match were fighting.
Tachikawa calls Kumagai by her nickname "Kuma". However, he does so because, in spite of having been corrected by her six times in a row in the past, he is unable to read the characters in her surname, and Kumagai has come to accept it.[BBF Q52] In his commentary of Nasu Unit's match against Tamakoma Second and Suzunari First Tachikawa bluntly stated that feelings hardly impact the outcome of a battle when Kaho Mikami praised Kumagai's resolve, but he later pointed out that thinking the opposite would equate to holding that the loser did not want the victory enough, implying that it would be a disservice to Kumagai, whereas he was actually quite impressed by her passion.
- "Just a few seconds. That's all I need. Just a few seconds. If I can only stop Murakami..."
- "Of course this wouldn't be that easy."
- "He'll do it. He'll make the move that'll tip the balance. I know he will..."
- "This is the first time I've really used a shooting Trigger. Imagining Rei's fighting style helps me move easier."
- "Keep hammering him just outside his range until the Raygust cracks!!"
- "Yūko" (友子? which can also be read as "Tomoko") means "friendly child", while "Kumagai" (熊谷?) means "bear valley".
- Kumagai likes apples, meat udon, and all kinds of sports.
- In her free time she enjoys playing basketball with Ryō Utagawa and Kuniharu Kakizaki, among others.