Amiguito

Recall that the diminutive suffix -ito can have several effects on a word:

1. Emphasize smallness: una cajita, a small box.
2. Reflect an attitude of respect, affection, contempt or embarrassment on the part of the speaker: su abuelita, her grandmother (polite).
3. Alter the meaning of the word: paloma: dove → palomita: popcorn, checkmark.

Amiguito

The word amiguito can demonstrate all of these effects. Mi amiguita, my dear friend. A parent could refer to her young daughter's friends as her daughter's amiguitas, emphasizing both size and affection. If you don't like your brother's friends, you could call them his amiguitos, implying that they are not worthy of his friendship or that perhaps they are not true friends after all. Tus pinches amiguitos. Your pesky friends. Context is your guide.

Friend with Benefits

Amiguito is also one way to refer to a friend with benefits, someone with whom you have an ongoing sexual relationship that is only casual. ¿O simplemente una amiguita? Or just a friend with benefits, not an actual girlfriend? I've also heard amigo con derechos to describe this kind of 'friend'. Amigo cariñoso is another euphemism for this, emphasizing the presence of feelings, a romantic element.

Some vocabulary borrowed from English also features here. Saying that someone is un sexfriend or un fuckbuddy emphasizes the sexual aspect further. Such a person could also be called un free, meaning that both of you are free to see other people, no strings attached.

Friend Zone

En plan de amigos, as just friends. Yo solo puedo estar contigo en plan de amigos. I could never see you as more than a friend.