My DS2 was the exact opposite for DS1 from the moment he was born.
DS1 came in kicking and screaming after quick labor, 6lb 5oz of outrage at being born. Brand new soul, took a while to learn how to nurse, was a total "by the book" baby, independent sleeper from 10 days old, active, cheeky, coordinated, loud, sporty, incredibly trusting, challenging toddler, tantrummy, and very anxious as a young boy. He's nearly 9 and very much feels his feelings and became far more cuddly from about 3-4 years onward as he started to slow down a little. Before that he wouldn't sit still long enough for a hug!!
DS2 arrived after a slow burn labor, 6lb tiny baby was placed on my chest, looked up at me with wide eyes, took in his surroundings, set up camp on my boob and didn't leave for the whole day. Old soul. A total smiling observer as a baby, rarely cried, hungry All. The. Time. CO-slept because he nursed through the night - now he's 6 1/2 and he's still sneaking into my bed at night, curls up with me like a puppy.
He isn't particularly sporty, is far more imaginative, gets on with everyone, laid back, loves and protects his older brother, questions absolutely everything, is charming and can floor us with the adult humor and gestures that he has absorbed from TV we assume.
They couldn't be more different my two, but my God if they don't love each other fiercely.
I love that they're completely different because there is no comparison. They compliment each other wonderfully!