Help! New Character!

So I'm adding a new character in in the middle of a book and they're going to be really major. But the reader is already attached to the characters I have so far and I don't know how to make them like thsi new character so that what happens at the end is really dramatic . Does anyone know how to make a character REALLY REALLY likeable?

You just gotta sell it! Their personality has to stand out from the others. What makes them unique to the already-made characters? What traits does this character have that the others don't? Then build off of that.

Wondering on if people like the character shouldn't be the focus when making it. Instead, think of what role this character plays, then have that character play that role well. Make them seem like the perfect character for their position. Build off of that relationship between the character and their role in the story. Then the personality will come out.

And of course, just make sure if this character is really needed! If you need this character to step into the story, then they must have an important role that the other characters couldn't play. And again, build off that relationship between the character and what they're there for.

Try making the new character and one of the old characters have a disagreement. Show it from both of their perspectives, and make the new character right, and the old one wrong. The old characters then start criticising the new one based on their standing, until a piece of evidence shows that in fact the new character was right. Then the old ones will have to be all apologetic. This should work because humans have a strong sense of justice and will sympathise with someone who's in the right, even though everyone else stands against them.

Thanks everyone! Case closed ^^ thank you so much for your input and I'm going to use all the ideas I got. Thank you!

You can also be really sudden with the new character introduction. One book I read introduced a major character a third of the way in, by having him be in the form of a bird and basically being chased into the vicinity of the other characters. He slowly grows into his major character role, and he just gets added to the list of major characters. He was more a character like Jabs described; one who fit the role perfectly and couldn't be replaced. He ended up becoming the MC's mentor because he knew the most about the MC's style of magic.