After Teddy got over the initial insecurity over James' birth (to read in detail read The Big Brother) , it took him no time to befriend James. Harry remembers when James had just learned to walk , he used to walk around the whole house , bumping into things. On one such incident , Teddy was present. When the baby stumble over thin air and fell down on the ground. His mum , his dad and his big brother all ran over to him in an instant.
Harry and Ginny rubbed his back and leg , where he got hurt, but James was still crying. So Teddy, always the saviour, took Jamie's leg and kissed his hurt shin. But the tears were still coming . In moment of desperation, Teddy looked down at the floor , slapped it and yelled , "You bad floor! How dare you hurt my little brother like that?"
He was about to slap it again when James' tears stopped, he cracked a smile and crawled over to his big brother's lap. If he were a cat , Harry thought, He would have probably purred.
Harry remember the time James decided that he and Teddy were best friends. Al had just been born and James didn't get over his insecurity as easily as Teddy had. Even though he had accepted his brother in the family, he still had trouble sharing. Whenever Ginny would be attending to Al , James would get sad and weepy and everytime Teddy was there. That's why, months after Al's birth. James had declared in a Weasley-Potter gathering that Teddy was only his best friend.
"I can share mum and dad even my toys with Al but Teddy is only my best friend. Al doesn't get him." said James without even asking Teddy's opinion on the matter.
A much older Teddy had just smiled at him and agreed with everything he said with half embarrassment and half amusement.
As a solution he also added that , "Al can be friends with Rose , since they are both gross weepy babies." Harry remembers laughing a lot that night.