Being friends with an anime protagonist is more often than not a challenging and life-threatening existence. In extreme cases, the best friend will get killed off as a tool to push the lead character to reach their potential. Even those who live, however, are often stuck in a thankless role.
A perfect example of such a thankless sidekick is Krillin from Dragon Ball. He initially sees Goku as a rival when they begin training under Master Roshi. Soon enough, though, they bond during their training and quickly become the best of friends. Even though they start on the same footing, Krillin will never catch up to Goku. The in-universe reason is that he's human, but from a writing perspective, all of Krillin's weaknesses serve to enhance Goku's strengths further.
Be that as it may, it doesn't mean that Krillin is the weakest character in the series. Because in comparison to other human martial artists, like Mr. Satan, Krillin is a strong fighter. But in the world of Dragon Ball, the villains are rarely human. So when the going gets tough, and it does get tough, Krillin can't always get going because he's usually one of the first people that gets beaten up, which either ends up with Goku saving him or Goku getting angry and powering up. Character dynamics similar to Krillin and Goku include Koichi Hirose and Josuke Higashita from Jojo's Bizarre Adventure: Diamond is Unbreakable, Noriaki Kakyoin and Jotaro Kujo from Jojo's Bizarre Adventure: Stardust Crusaders and Armin Arlert and Eren Yeager from Attack on Titan.
Thankfully, despite being one of Goku's weakest team members, Krillin's personality properly develops, and he's even happily married to Android 18. What's more, he's still part of the cast until the latest installment in the series. Some best friends aren't as lucky, though, and end up fading into the background until they're entirely gone. One such best friend who suffers the fate as mentioned above is Naru Osaka from Sailor Moon.
At the start of the series, Naru is a staple character in Usagi's life, as her closest and best friend. She's also almost always involved when villains wreak havoc and become Sailor Moon's 'damsel in distress' on many occasions. Naru does get a shot in the spotlight when she falls in love with the villain, Nephrite. However, the focus on her doesn't last long since her relationship is short-lived. After Nephrite's arc ends, Naru's slowly paired up with another supporting character to somehow give her story a sense of closure.
By Sailor Moon R, though, she's not even part of the regular cast anymore but has been demoted to a minor character. Given that she still makes several appearances, it feels more like a cameo because she's no longer relevant. In Sailor Moon S, she barely shows up, except for one episode where she and her boyfriend, Umino, are highlighted because the villain attacks them. And finally, she's briefly seen for the last time in one episode of Sailor Moon Super S. When the final season, Sailor Moon Sailor Stars, starts, it's as if she never existed.
Naru's involvement and importance to the story gradually lessen, and as a result of so many sailor senshi popping up, there isn't any screen time left for her. Sad as it may be, this is one of the typical paths laid out for the hero or heroine's friends. Her part in the remake, Sailor Moon Crystal, doesn't get any better, stuck as an easy to ignore minor background character. Some characters who went through the same process are Princess Cornelia Ii Britannia and Jeremiah Gottwald from Code Geass, Misa Amen from Death Note and Lotte Jansson from Little With Academia.
The ones who have it the worst are the best friends who ultimately die after making such a significant impact on the story's hero, just like what happened to Kamina from Gurren Lagann. The shy and cowardly Simon saw the confident Kamina as a friend and role model with whom he easily connects. Unfortunately, after a devastating battle, Kamina dies, but not before he helps Simon for the last time, solidifying his tragic best friend status. Other best friends who go down the same road are Seung-tae Woo from God of High School and Eugeo from Sword Art Online: Alicization.
However, it's not all bad for anime best friends. In rare cases, they can turn into main characters, too, like Killua Zoldyck in Hunter X Hunter. Gon might be the main protagonist, but from the first moment Killua interacts with Gon, it's apparent that he's not here to be a sidekick or play second fiddle. While there are slight differences in their strengths, but it's just a matter of time before they even the playing field. They're not only best friends but friendly rivals, pushing each other to hone and improve their skills. Killua becomes such a vital and irreplaceable character in Hunter X Hunter that it's not uncommon for fans to favor him over Gon. Killua shows being the hero's buddy for life can have its upsides.
