I have Norman’s lampeye killifish, they’re small fish but do need a school. They have cute reflective blue eyelids but the rest of their bodies are plain in color making it less likely they’ll aggravate your Betta. Still comes down to the individual Betta for temperament.
Responders….how can anyone answer this question before asking how big her tank is???? No additional fish in a tank less than 10 gallons. And in a 10 gallon I’d only tey one or two guppies (they are very “dirty” fish and produce a lot of waste). Whether your betta will tolerate tank mates is another matter. Each has his own personality.