He is a late bloomer, i have not seen many of his games but he is very skilled and is more of a playmaker but hes got a great shot too from what ive seen.
Found this description on Eliteprospects:
Keränen is a skilled offensive minded forward. A fast and flashy dangler who likes to challenge the opponents. Able to set up his team mates with deft passes, but also to score by himself. Not a slouch defensively, but his talent goes to waste as a bottom six. A bit of a late bloomer.
It's not up to date because he has proven to be a top 6 player this year, im sure he would do good in the KHL but if he wants to play in North America he needs to get some more muscle.
I've seen him many times in finnish league. Keränen has become a much better player during his four seasons in liiga.
The first time his name came up to my knowledge, was in 2009-2010 season, when he had a strong season with A-junior team. Those who watched A-junior games, reported he has talent to become offencive role forward in Sm-liiga. He was reportedly a sniper type of player in the A-junior league.
Some sources have listed him as a center, but he played as a right wing already in A-juniors.
In his rookie season, 2010-2011 he played just in the fourth line when he was with the big club. He didn't impress much and didn't get much ice time.
He already had a decend buck control in his rookie year and seemed to often win board battles despite his thin stature.
Ilves had a young team in that season, but other youngsters like Rajala, Rask, Sointu and Myttynen were ahead of him in ice time.
In 2011-2012 season Keränen was given more ice time and he played also as a second line center besides playing in his usual position at right wing. Keränen didn't show much despite the better ice time and the team with a weak forward lineup finished last in the regular season. Looked like he needed more time and space to score more points.
In the 12-13 season he made his big jump and looked like a very good player on the ice, playing somewhat aggressive and physical, hard working and hard skating game, nearly always winning in 1 vs. 1, winning board battles and loose pucks. He worked hard always and played well in both ways.
Keränen was mainly played in the third and fourth lines during the 12-13 season, but managed to show of offencively as well, scoring 27 points.
Keränen is a good and pretty fast skater. However he is not like Gaborik fast. Rajala, Rask and Sointu are faster and more agile skaters, than Keränen is.
Keränen has good balance and seems like strong on the ice. He seldom looses board battles and he is very strong on the puck, controlling puck very well. Besides his excellent puck control he has very good hands by Liiga standarts and he is a very good deker.
Keränen also has a very good slap shot and a very good wrist shot by liiga standarts but his primary weapon is his playmaking and passing skills.
Unlike some skilled players Keränen is not passive neither flegmatic and he plays somewhat aggressive game with some energy and works hard both ways. He seems to skate hard and work hard all the time and when he is with the puck he is creative and dangerous.
Keränen is not a hitter, but he plays the body.
I think he is two steps away from being an NHL player. He'll take the next step in a year or two, but not sure will he ever make the NHL. If he wont be an NHL regular, he will have a good career in the KHL.
Keränen seems to be strong mentally and seems to work hard to become a better player.