The protagonist of most shonen anime usually has more than one official sidekick and friend. In contrast to Robin in Batman, these characters are the shonen hero's genuine equals in many aspects, and they may have a friendly or hostile rivalry that brings out the best in both of them.
Both Bleach and My Hero Academia provide their main characters with a tough-talking tsundere sidekick who has great chemistry and drama with them. Izuku Midoriya's childhood buddy Katsuki Bakugo, a classmate and rival at UA, and Rukia Kuchiki, a critical ally who taught Ichigo the fundamentals of Hollow hunting. These tsundere sidekicks are important to the plot but have a serious attitude, and one of them is particularly interesting.
Rise of Bakugo as Midoriya's Flamboyant Rival in MHA
Katsuki Bakugo, in contrast to Midoriya, was born with a potent Quirk that enables him to trigger explosions with his nitroglycerin perspiration. This, combined with his admiration for All Might, motivated Bakugo to strive for the top and put in a lot of effort to become the next top hero. As a trainee hero, Bakugo is infamous for being aggressive, competitive, and self-centered. He frequently behaves more like an antihero who uses the wrong tactics to fight for the right cause. Although he plays the sidekick in My Hero Academia, he frequently rejects the label.
Bakugo can't overcome One For All's power though, and he knows All Might favors Midoriya over him. So, after being in denial for some time, Bakugo gradually came to terms with being the protagonist's #2, fighting alongside him to help the latter realize his destiny to defeat Tomura Shigaraki. Bakugo is now a young Endeavor, accepting that he wasn't chosen by fate but still playing a key role in supporting the hero. Though reluctant to be in this position, he's still proud to be Midoriya's best ally and worthy rival, even at the risk of his own life.
How Rukia Became a Friend and a Bridge Between Worlds for Ichigo the Soul Reaper
Unlike Bakugo, Rukia Kuchiki wasn't Ichigo's childhood friend in Bleach, since she grew up in the Soul Society and was born decades before he was. The two met one fateful night in a reverse isekai adventure in Karakura Town; facing the Hollow called Fishbone D., Rukia gave Ichigo her Soul Reaper powers, then adapted to human life in her gigai body and mentored Ichigo in the ways of Hollow hunting. Over the course of Bleach, Rukia went from a captive in the "Soul Society" arc to a reborn Soul Reaper who fought by Ichigo's side as an equal. She grew and gained new powers at the same pace as Ichigo, showcasing her new ice-based shikai before eventually unveiling her bankai, Hakka no Togame.
On a personal level, Rukia had famously strong on-screen chemistry with Ichigo as his tsundere friend and sidekick despite having only met him recently. She was sometimes playful and whimsical during her stay in Karakura, and other times, she scolded Ichigo or tearfully sympathized with characters like Orihime, Yuzu, Karin and more. She also fought to overcome her own guilt about Kaien Shiba's death, first apologizing humbly to Kukaku before overcoming her personal demons when she fought Aaroniero Arruruerie, who had disguised himself as Kaien. Rukia also made up with her foster brother Byakuya, and the two of them became much closer.
Who Makes a Better Tsundere Sidekick: Bakugo from MHA vs. Rukia from Bleach?
Both Bakugo from MHA and Rukia from Bleach have compelling appeal as tsundere sidekicks and compelling personal arcs that bind them tightly to the hero. Rukia experienced significant losses and suffering before coming to terms with everything, but Bakugo was an annoying and gruff antihero who gradually learnt to accept his place as Midoriya's sidekick while giving up his goals of becoming #1. Rukia, though, stands out as the most admirable tsundere supporting figure.
Rukia matches Bakugo in many ways -- a surprisingly deep and multi-faceted personality and the steady growth of powers -- but she has even stronger chemistry with the shonen hero and a more nuanced character arc overall. Rukia overcame more hardships than Bakugo ever did, such as her foster brother's emotional distance and her childhood poverty in the Rukongai District's streets. She is also lovable for being a classic reverse isekai hero in an anime that often toys with those elements in creative ways. Rukia is a girl of two worlds with friends in both, and she made serious personal strides when she reconnected with Byakuya, overcame her grief and guilt about Kaien's death, and more.
She balances her tsundere rage with unreserved generosity for others, making her more compassionate and protective than Bakugo. Bakugo from My Hero Academia, on the other hand, is outright evil with his own tsundere attitude and can be sickeningly abusive and self-absorbed toward others. Aside from using Quirks, he rarely aids other people. Even if Bakugo has now apologized for everything, the harm has already been done. Rukia from Bleach is a far better role model because she is a loving tsundere sidekick who everyone can look up to.