Questions & Answers about Hei, kuka sinä olet?
Why do we use kuka instead of mikä here?
Kuka means "who" and is used when asking about people. Mikä means "what" and is mostly used for things. Since you are asking for a person's identity, you must use kuka.
Why is the word order "who you are" (kuka sinä olet)?
In Finnish, questions that start with a question word like kuka keep the normal statement word order right after it. You don't swap the subject and the verb like you do in English, so the order is Question Word + Subject + Verb.
Can I drop the word sinä?
Yes! Because the verb olet ends in -t, it already tells us the subject is "you". Saying Hei, kuka olet? is perfectly correct and very common in spoken Finnish.
Is this polite enough to use with an older stranger?
Sinä is informal. It is fine for casual situations or people your own age, but if you are speaking to someone much older or in a formal setting, it is more polite to use the formal "you" and say Hei, kuka te olette?.