Small physique, uses his low center of gravity to his advantage well.
Exceptional passing ability, short or long. I don't know if there are youtube videos of him but I'm sure you know how to find them if there are.
Once again, exceptional technique.
Pace-wise, I would consider him on the slow side, perhaps even very slow by EPL standards.
His playing style is best suited in a 4231 (that Mancini has adapted and brought to City when he first came) sitting in front of the back four with a bulldog ball-winner type beside him. That's not to say he can't defend on his own, but for a typical defensive midfielder it is at best average and has steadily decreased with age. He is very creative, perhaps too much so at times. He likes to do quick turns to pull away from defenders instead of just kicking it away. It's comical to watch him do it to strikers that are chasing him around. For this reason he is very good at keeping possession and completely controlling the tempo of a match. He's very comfortable and confident on the ball. He is able to draw fouls very easily with his turns. Some may consider it diving, since the contact is only sometimes enough to knock him down, but he will go down if there is contact. This might be an issue transitioning from Italy to England. Referees might not be as lenient or protective, but at the same time, he may be able to draw even more fouls because the league itself. This puts a lot of strain on his body.
100% fit he is without a doubt a difference maker on the pitch. Perhaps not as flashy as the winger/attacking midfielders constantly running at defenders, he makes space for himself before finding someone who is open. The drawback here is that he has been struggling to be 100% fit for nearly two seasons now. Some argue it's the same injury that he's failed to recover from while others say he's turned brittle. Maybe a new medical facility can make a difference. Him being less then fully fit has a huge impact on his performance. His turns and dribbling are extremely limited and he turns into a very weak "holding" player because of it.
His personality is very soft-spoken. There shouldn't be much drama from him. Although he did walk out on training once this season, later saying it was a family emergency (and then returned to Chile). Nobody knows the full story behind that. He's not a vocal leader, but possibly a silent one. I would still consider him very professional.
Comparable players would be Xavi or Pirlo, but with more defensive capabilities. I consider Pirlo a level above the three because he has been great for a number of years while Xavi has only excelled since Pep made Barcelona perfect for him and for that has only been known for a few years despite being nearly the same age. I'm not saying he's better than Xavi or anything, but you won't have the arrogance or Barcelona diva in him. If healthy, he can be a gem. The big thing will be how he adapts to the league and how he is played by Mancini.
Sorry it took so long.