@Serenata
Well, appearancewise broadly speaking, there aren't that many different types possible in isekai stories.
But generally, they're either someone near the age of just becoming an adult(just before 20s or just into), thus new to adventuring and joining around the same time as the MC, thus they can't really have that really different appearances, or they're in their mid 20-s after advancing in their profession for some time. Because we've all seen how new adventurers starting out in their 30s or 40s are treated, and women would probably have it even worse.
Frankly, actual MILFs and somewhat mature ladies as part of isekai harems are maybe 90% non-humans who've then transformed into a humanoid shape. There just aren't that many opportunities, routes or reasons for their types to end up with the 'fresh into their adulthood' type MCs.