"니" is a dialect of "너" as Gherelmaa said above, but it is also used as the spoken form of "너의" or "네가". e.g. 네 옷 => 니 옷 네가 => 니가 \^o^/

What matters is just pronunciation, 니 is an other way of saying 네(<너의) because Koreans from some regions other than Seoul don't know the pronunciation difference between '내(<나의)' and 네(<너의)

I guess, you can only use 너 when first time addressing a stranger in the following situations: 1) in a friendly manner - if you are a child and you are addressing an other child supposedly of your age or younger; 2) in a rude and aggressive manner - when you are strongly unsatisfied with his/her actions and are ready to start a fight with that person ~~

and when I meet a strangers, first time to talking with they can I use "너"

so "니" mean "your" not "you"??? so how to tranlate "니가필요해"