Colloquially, and in Taglish, "girlfriend" is what we use.
KASINT
AHAN is the true Tagalog word for girlfriend/boyfriend but nowadays it is only used in formal conversations.
NOBYA is adapted from the Spanish "novia". Older generations still use the word.
DY
OWÀ or J
OWÀ is slang for boyfriend/girlfriend/lover/live-in partner and is in common use among Millenials and younger generations.
SY
OTÀ is an older slang term for it and is not as popular as the more encompassing "dyowa/jowa" anymore.
"Girlfriend" is your best choice. If you really want to say it in Tagalog and you are not younger than Gen X, you may use "nobya". If you're a Millenial or younger, "jowa/dyowa" is OK, but use it only among peers. Never use "jowa/dyowa" or "syota" to refer to your girlfriend when talking to her parents unless you want to be branded as Mr. Crude. 🙂