Trini is the original Yellow Ranger.  Zordon chose her because of her "lightning hands and peaceful soul."  Quite lovely, don't you think?   Always quiet and observed (but don't push her around; she'll kick your butt).   Trini is probably the one who avoided the most spells.  Let's see, Jason and Tommy were turned against each other, Billy and Kimberly were turned into punks.   Yeah, Trini and Zack both avoided more spells.  When they did fall under spells, the other Rangers fell with them.  Trini was especially helpful because she understood Billy's computer language.  Without her around, Billy would not have been very helpful.