Arabic only uses the word for ‘who/which’ (ílli) when it refers to a definite noun (i.e. ‘the …’):
ána shuft il-bint ílli káanit I saw the girl who was rákba HuSáan. riding a horse.
ána shuft bint káanit I saw a girl [who] was rákba HuSáan. riding a horse.