"Buddy" is a special way to use monsters during the game. At the start of the game, each player chooses 1 monster from his/her deck and places it into the buddy zone, then that monster (and all other cards with the same name) becomes his/her buddy. The buddy monster does not count towards the 50-card minimum deck size, nor does it count towards the "4-per-deck" rule. This means you can have a 5th copy of your Buddy Monster for place in the buddy zone.
It is possible to use a non-monster card as buddy if its ability allows it to be placed in the buddy area at the beginning of the game, as well as gain 1 life with buddy gift.
If the player would call a monster from anywhere with the same name as his/her buddy, he/she can reveal it and declare "Buddy Call".
When Buddy Call is declared, the player puts the monster in the buddy zone to a monster area at stand and then place that another copy revealed into the buddy zone at rest. If the Buddy Call was successful (it was not nullified), the player gains 1 life with "Buddy Gift". You may only perform Buddy Call once per game. The Buddy Call is considered a single call, and if the monster has "Call Cost", it must be paid first. After the player performs Buddy Call, the card placed in the buddy zone stays at rest for the rest of the game.
Also, you may buddy call using effects that call monsters, such as Quick Summon. In that case, call the buddy as normal and then place the monster called by that effect in the buddy zone at rest, you still gain 1 life.
All copies of your Buddy monster are treated as the Buddy. If a card ability involves the Buddy such as Ultimate Buddy!, it can apply with any copy of the Buddy monster even if it wasn't buddy called.
If you do not have at least 4 copies of your buddy monster, you may use a My Buddy! as a substitute and you may Buddy Call using it. In order to do it, the My Buddy! must be in your hand, if it is, declare Buddy Call, then place your Buddy Monster from the buddy zone to a monster area at stand, and place the My Buddy! from your hand into the buddy zone at rest. Remember that you may only Buddy Call once per game, even using My Buddy!.