Pisi, the younger brother of Aviva Premiership Rugby Dream Team centre George, has risen rapidly through the Samoan representative ranks over the past few months, from taking the IRB Sevens Series by storm to making his full international debut in last Saturday's 20-18 Pacific Nations Cup win over Tonga.
The 23-year-old - 6 feet tall and weighing 89 kgs - also came off the bench in Samoa's Round 2 win over Fiji and is comfortable in any of the outside back positions. He will bring pace and physicality to the Saints' attack when he arrives at the club once Samoa's international commitments have been completed.
Although capped by Samoa, Pisi's early representative career was in New Zealand's colours, and he played against two of his future club mates - Calum Clark and Courtney Lawes - in the 2009 IRB Junior World Championship final.
After five years at ITM Cup outfit North Harbour he is excited by the prospect of moving to Franklin's Gardens.
"I want to experience something different," he said. "I was impressed by Jim Mallinder and the rest of the Saints coaches, and I've seen that it is a good environment, has a strong sense of community and is a club which takes care of players.
"Obviously the fact that George is doing well at the club made my decision easier. He's been enjoying his rugby and told me that the club is well organised with a tight squad, and everything that I've seen myself backs that up.
"I like to think that I'm an attacking player, so I think I'll fit into the style of play. The Saints like to use the ball and to attack and are an exciting team to watch. I want to fulfil my potential and am looking forward to doing that in the black, green and gold of the Saints."
Director of rugby Jim Mallinder says that Pisi will be a good addition to the Saints squad ahead of another season of Aviva Premiership Rugby and Heineken Cup action.
"We've been very excited by what we've seen from Ken," he commented. "He is fast and physical and has good rugby awareness and handling skills. He is also versatile and can slot into a number of positions, which is important for us as we try to maintain a squad which can compete both in the Premiership and Europe while staying within the salary cap.
"We think that Ken will fit into our style of play and am looking forward to welcoming him to Franklin's Gardens in the next few weeks."