Apparently that's female trainer because this is tagged Umamusume 'Yuritty' Derby on pixiv

Cpt_Skippy said:

Apparently that's female trainer because this is tagged Umamusume 'Yuritty' Derby on pixiv

That tag has since been removed. In a comment the artist described the trainer character as "中性" which I suppose could be translated to "neutral gender" so whether it's a male or female trainer is up to interpretation I guess. I changed the tags accordingly.