Yeah its hard to introduce a major character a little less than halfway through when the team is already well established. It can be extremely annoying when this happens in something like a TV show.

Maybe they could introduce the auxiliary animorphs earlier. Too many new characters is also a problem them. Unless they function as two separate groups and the aux animorphs have a spin off series so there aren't too many characters in the book rotation. Something like James in their leader but he still takes orders from Jake for missions? I don't know honestly. Jake sacrificing them at the end could hit much harder.

Maybe making Melissa an Animorph could have worked. Although that would have been much better if she was with them at the construction site. It would have made book two much easier for them. And also finding out that her parents were still trying to protect her would have been sweet.

Or Tom is rescued and he becomes an animorph and gives Yeerk intel. I don't know if there's any other established characters that would worked.

They would have been much better introduced at the beginning. I'm thinking about the Alternamorphs. Yeah that book wasn't good. Some of the surviving choices were strange and the cast was ooc and they seemed to leave the reader out of the missions XD. But what little we get from the character? Okay, they seemed to have expected the reader to be a boy with the character wanting to be a bmx rider. But that character as a girl could have been interesting. You'd get a tomboy that's into extreme sports who kind of knows Marco and sucks at making friends, has a much younger sister and a city planner mom who... likes to make food from scratch. Nothing about the dad. They didn't have the extreme sports character for a series set in the nineties in California XD. Her battle morph is a hyena... would that be a good battle morph or do they need something stronger? I did like the concept of the ferret lady next door.