Benvolio and Romeo: They are cousins, but more importanly friends. Benvolio is the "peace keeper" in the play and he wants nothing more than for Romeo to be safe and happy. In the beginning of the play in Act I, he tries to get Romeo to get over Rosaline and "examine other beauties" (Act I, Scene i, line 230- see link 1). He also tries to stop the street brawl in Act I and the fight that ends in the death of Mercutio and Tybalt in Act III Scene i. This is a typical friend relationship.
Romeo and Juliet: Teenage lovers. They meeting in Act I Scene v and it is "love at first sight" (or perhaps lust). They profess their love and get engaged later that same night and are married the next morning. They kill themselves for each other at the end. This is a typical, albeit quick-moving and rash, romantic relationship. Look mostly at Act I Scene v and Act II Scene ii (see link 2)
Friar Laurence and Romeo: Friar Laurence is a mentor to Romeo and is constantly giving him advice. Key passages are Act II, Scene iii, lines 66-81 and Act III, Scene iii, lines 114-16. This is definitely a mentor/mentee relationship. (see link 3)