The most accurate and natural rendering of in solid text (without spaces or capitalization changes) is:
If the context is a name or title, it is often written as one word in English transliteration: yama no hime
Meaning: Mountain Princess / Princess of the Mountains The most accurate and natural rendering of in
However, if you need to preserve the original romanized Japanese word boundaries while using solid text (i.e., no spaces but clear reading), the standard way is: no spaces but clear reading)