(CNN) He's known as "The Special One," but now one of Chelsea's star players has revealed a more complex picture of Jose Mourinho.

Diego Costa, the English Premier League champion's leading goalscorer last season, says the Chelsea manager has a softer side which few outside of the locker room ever get to see.

"He is a person that lives, loves, fights and works -- he is always trying to improve," Costa told CNN when asked about the 52-year-old.

"He is that person that you see with the ambition to be victorious, the ambition to win, the ambition to become better.

"That is a point that makes the difference -- that ambition to be better than you were last year."

Mourinho led Chelsea to the English title last season after returning for a second spell in charge at Stamford Bridge in 2013.

The Portuguese coach is one of the most successful in the game having won two European Cups --first at Porto and then at Inter Milan -- while he has claimed domestic league titles in England, Italy, Spain and his native Portugal.

.@hazardeden10's first goal in a Chelsea shirt was against Seattle Sounders during our 2012 #CFCTour of the US... https://t.co/KW9W6TGEw0 — Chelsea FC (@ChelseaFC) July 22, 2015

While Costa talks about Mourinho's more genial nature and his "friendship" with players, the Chelsea manager is not without his detractors and the Portuguese's behavior outside of the home locker room has often been criticized.

His touchline spats and arguments with referees are well documented, notably an incident with the late Tito Vilanova -- the former Barcelona manager -- who Mourinho poked in the eye after an argument during his time at Real Madrid.

No stranger to controversy himself, Costa refrained from commenting on Mourinho's past transgressions, instead preferring to praise his coach's more personable nature.

"He is a very intelligent coach," Costa said ahead of Chelsea's exhibition match with Major League Soccer's New York Red Bulls.

"He gives a lot to the players, he is a coach that the players are very close to, which is a thing that is also very interesting -- his friendship.

"He is always in the locker room, he is always joking, he is not this serious person that everyone sees outside of the pitch.

"He is a person, who is excellent for us inside the locker room. I think, like I said, that for us he is the best. For people outside its just his way, I don't want to get into that.

"While he continues to be what he is with us, that wonderful person, and the excellent coach that he is, that is great."

One of the accusations leveled at Mourinho and Chelsea last season was that its brand of football was boring.

While Barcelona thrilled in Spain and Bayern Munich did likewise in Germany, England's champion was often accused of failing to entertain.

And Mourinho was frequently forced to defend his side's style of play as Chelsea moved closer and closer to the league title.

"A season is very long and there will be periods where you'll be very good and moments where you won't be as good," Costa said.

"I think that what's important, and that's why I say our coach is intelligent, in a moment when you're not as good, he chooses the team in a way that minimizes our chances of losing any points.

"The season is long, players get tired, there are moments when players are in poorer form and it is in those moments that he acts."

Chelsea will face a severe challenge to retain its crown in the coming year with Manchester City, Manchester United and Arsenal all expected to challenge the champions.

In particular United, which finished fourth last year, has recruited heavily in the offseason, buying Germany World Cup winner Bastian Schweinsteiger from Bayern, France international midfielder Morgan Schneiderlin from Southampton, $34 million forward Memphis Depay from PSV Eindhoven and defender Matteo Darmian from Torino.

"I think the biggest opponent will be ourselves," said Costa, who was born in Brazil but plays for Spain.

"If we make but a few mistakes and if we do what we have been doing and work how the coach wants us to work, I think we can win the title again.

"The biggest opponent will be ourselves, the players, individually, each of us wanting to be better than we were last season."