There are several interesting characters from One Punch Man that have yet to be introduced in the anime adaptation, with Suiko being one of them. Unfortunately, her character is unlikely to be adapted in the anime in the foreseeable future.
For those unaware, Suiko is a martial artist and the younger sister of Suiryu - the arrogant yet ridiculously talented fighter from the Super Fight Tournament arc, who got a harsh reality check after being utterly destroyed by Gouketsu and Bakuzan.
Like her brother, Suiko is also trained in the Void Fist style, i.e., a martial arts technique which focuses on maximizing the destructive power of one's attacks while simultaneously maintaining a consistent output of strikes.
Suiko first appeared in chapter 141 of the One Punch Man manga, almost near the Monster Association arc's end. Considering that the anime hasn't even adapted the first 10 chapters of the Monster Association arc, fans shouldn't keep up hopes of seeing her debut in the anime anytime soon.
Disclaimer: This article contains spoilers for the One Punch Man webcomic. Reader's discretion is advised.
While she hasn't reached that point in the manga yet, in the One Punch Man webcomic, Suiko eventually became a professional Hero and joined the Hero Association. There, she became an A-Class Rank 40 Hero.
Moreover, in the webcomic, Suiko is one of the few people aware of Saitama's true strength, which she discovered when he saved her from a battle robot called Mirror. She also acknowledged that he was leagues stronger than she and her brother, Suiryu.
There's not much to unpack about Suiko character-wise, especially due to her minor part in the narrative at the moment. Despite being mostly her own person, she does have a few things in common with her brother.
Suiko has an easygoing and carefree personality, often even coming across as laid-back. Like Suiryu before his wake-up call in the Super Fight Tournament arc, Suiko can sometimes be a bit arrogant and tends to overestimate her own abilities.
However, Suiko isn't a complete narcissist either, like her brother in his earlier days. She recognizes that there's always room for improvement and is actually willing to put in the work to better herself, showcasing more self-awareness than her brother.
What makes her character especially likable is the fact that, despite her overwhelming confidence, she doesn't look down on others. Unlike Suiryu in his earlier days, she knows how to acknowledge those stronger than her and doesn't need a harsh reality check to come to her senses.
How relevant is Suiko in One Punch Man?

To put it bluntly, Suiko isn't exactly one of One Punch Man's most relevant characters. She doesn't affect the plot directly and has been given extremely limited screentime in the manga. Unlike her brother, she hasn't been given any major moments that push the main plot forward.
Suiko isn't a part of the core cast in One Punch Man, and at the moment, it's hard to say whether she will be given a major role in any of the upcoming storylines. Compared to characters like Genos, Blast, Flashy Flash, or even Speed-o'-Sound Sonic, Suiko hasn't been placed in the spotlight yet.
As such, fans of her character can only hope that the creator of the series, ONE, gives Suiko's character an important role to play in one of the series' future storylines, preferably before the series comes to an end. She's one of the series' most interesting side characters, who could certainly benefit from more exposure in a relevant story arc.
Final Thoughts
Suiko may not be in the main spotlight in One Punch Man, but she's one of those characters who certainly could be. She's competent, grounded, and overall an extremely likable character.
For now, fans can only hope her character gets more exposure in future chapters of the manga or the webcomic - maybe even an opportunity to fight monsters alongside her brother.