A certain name which ends with a consonant like 서현, 예림, 예슬, 택연 needs an additional suffix '이' before a subject marking particle '가', because it makes the sentence clear and smooth. In short, here '이' has no meaning but has just the funtion that makes the sentence clear and smooth. On the other hand, a name which ends with a vowel like 아이유, 미나, 지디 doesn't need the '이' so it can be combined with '가' directly.

I kinda disagree with that I read somewhere that 누가 is 누구+가, but to make it easier it become 누가. Similar case with 저+가=제가 or 누구+는=누군 words just change sometimes. Hehehe... :)

1) In the word '누가', '가' is not a particle. '누가' is equivalent to 'who' in English. 2) In the phrase "서현 + 이가", we add '이' to make it easy to prounciate