Choosing Their Battles

Thirteen's first condition was absolute trust and loyalty.

Even if the orders he gave were to seem like he was sending Cristopher to his death, the chubby boy was required to do it no matter what.

After Cristopher accepted this condition, Thirteen told him the other two conditions that the chubby boy needed to follow so that the former could continue traveling by his side.

Thirteen's second condition was that if they were to fight against any Monsters, Cristopher would always land the finishing blow no matter what happened. 

The chubby boy found this second rule strange. 

Although facing off against Monsters was dangerous, it was still advantageous for him if he landed the last blow on them.

Why?

It was due to Monster drops.

Items, Weapons, and Avatars drops would be obtained by the person who landed the killing blow on a Monster. This was an absolute rule for those who were chosen to become Wanderers.