Mariko is unique in the cast of Ranma ½ in that she is, while not the first girl to be shown as attracted to Kuno in the series (nameless girls are impressed with him during the Martial Arts Rhythmic Gymnastics storyline and after he gets amnesia in the Watermelon Island storyline), the first and only girl to actually try and seduce Kuno away from his obsession with Akane and Ranma's girl form. However, she canonically proves unsuccessful in this goal, as Kuno never really appreciates her feelings, and in the manga is shown as having become disillusioned with him after giving up on him.
Mariko and Kuno had a rather unusual first meeting. Right as Mariko finished a spiel about how she had been unable to find a man to love and how she wanted to meet the man of her dreams, Kuno comes flying through the air (having glomped Akane Tendo by surprise and received his usual punishment for it) and lands on her head. Abashed at having inadvertently injured her, Kuno places a bandaid on the injury, apologizes and walks away, leaving behind a starstruck Martial Arts Cheerleader. She promptly begins gathering photos of Kuno and unabashedly courting him. Kuno, however, receives all of her attention with a blase attitude, seeming to consider it his due, and makes no efforts to reciprocate it -- in part, at least, because Ranma Saotome is deliberately playing up to Kuno's delusions as an effort to make Mariko hurt for having defeated and humiliated Akane Tendo earlier in the story and their romantic moments are interrupted by school jocks or Ranma. This eventually leads to a Martial Arts Cheerleading match in which Mariko and Ranma compete to prove "who loves Kuno more", which Mariko eventually loses and decides to give up on Kuno. In the manga's comparatively longer ending, she is shown to be angry at Kuno and already obsessing over a new guy(the implication being Mariko simply has short obsessions on random guys every once in a while), while Kuno requests for help with his injuries falls on deaf ears.
Mariko's relationship with Kuno is different in the anime, in this continuity she seems more obsessed with Kuno (having a large poster of him in her room) and willing to overlook his flaws (is unphased by him falling for Akane's "Love kick of betrayal"). Unlike the manga ending, Mariko instead tearfully and very reluctantly gives up on Kuno, attempting to cheer herself up on the way out, still seeming to have feelings for him. Kuno does not react to this as he was still unconscious at the time.