Kille is the male analogue to tjej. It might be more accurate to say that the translation of "pojke" is "little boy." In English, it is still quite normal for even a teenager to refer to the opposite gender as a boy or a girl, but in Swedish, pojke and flicka are definitely not used much when referring to any post-adolescent person.
Here's how I think it is, someone can correct me if I'm wrong. When a vowel is followed by double consonants, it marks that vowel as a short vowel, i.e. short 'a' in hatt (hat) vs long 'a' in hat (hate). However, 'k' is never doubled (might be exceptions), but instead when marking preceding vowel as short, k is instead paired with 'c' as 'ck'. Examples, Short 'a' in macka (sandwich) vs long 'a' in maka (spouse), short 'u' in ducka (duck; as in keep your head down, not the bird) vs long 'u' in duka (set the table).