One common theme is power - how an OC's power interacts with Rio's. For example, in some fanfics, the OC has a power that complements Rio's fighting style.
One popular 'Super Sentai Gekiranger Rio' OC fanfic is 'Rio's New Adventure'. It explores Rio in a new storyline where he teams up with an OC. Another one could be 'The Secret of Rio', which delves deeper into his past and relationships with the help of original characters.
The " Rio " series was a low-alcohol fruit wine that was targeted at young consumers, especially young female consumers. There were five flavors in the series: lemon rum, grape Brandy, grapefruit vodka, yogurt vodka, and white peach Brandy. The alcohol content of these cocktails was only 3° C. The taste was sweet and smooth, more like juice soda. The purpose of the " Rio " series was to provide young consumers with a one-on-one drinking experience, allowing them to enjoy a relaxed and pleasant drinking experience in a state of solitude. The series also signed a popular celebrity, Zhou Dongyu, as the spokesperson. In general, the RIO series was a low-alcohol, multi-flavored fruit wine that was suitable for young consumers to enjoy a little drinking time in different moods.
An interesting plot could involve an OC who was initially an enemy, believing the Gekirangers were a threat to their own goals. But as the story progresses, they discover they have a common enemy. The OC then has to learn to work with the Gekirangers, overcoming their differences and prejudices. This would create a lot of character development and interesting interactions between the OC and the existing characters.
Give your character a distinct personality trait. For example, if your OC is always overly optimistic in the face of danger, it can make them stand out. They could be cracking jokes during battles or always seeing the good in every situation, which would be different from the more serious characters in the series.