I Miss Them Already 

"Master, I also want to go with you!" Taiga insisted.

He had been trying his best to make the younger boy accept his proposal because he wanted to be close to Shasha.

The Tigerkin was afraid that in his absence, his annoying love rival, Kane, would progress with his relationship with Shasha. 

However, Thirteen ignored him.

The boy didn't want to take responsibility for the consequences of bringing the Tigerkin to Pangea.

He also didn't know if the Laplace Demon would allow Taiga to step into another world because it was a place where someone like him wasn't supposed to go.

The other Monsters were different because people had already gotten used to Avatars, and such.

If a Tigerkin like Taiga were to appear, those with certain fetishes might kidnap him, and experiment on his body.

In short, Thirteen didn't want to deal with that mess, so he had no intention of bringing the persistent Tigerkin to Pangea.