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After making the code switch from the Warriors, Lousi initially struggled at the Waratahs but a move to Wellington has seen his game thrive since 2017.
In recent seasons, with the Lions and Hurricanes, Lousi has morphed into a much more rounded talent.
He has always been a dominant physical presence around the field but his strength with ball in hand is now complemented by skill in regularly holding up attackers to force turnovers. His work at the lineout and breakdown has also vastly improved.
Lousi, who is eligible for Tonga and could be selected for the World Cup, has not been sighted this season due to a pectoral injury.
Just as New Zealand rugby was beginning to see the best of the 27-year-old, he is now preparing to leave.
His departure is another blow to the Hurricanes who already lack depth in the tight five and will also farewell All Blacks Nehe Milner-Skudder, Jeff Toomaga-Allen and Matt Proctor – all of whom are heading north – at the end of the season.
"I am very excited to join the Scarlets for next season. It's a team that plays an exciting brand of rugby and I am looking forward to linking up with a group of players that are ambitious to be winning trophies," Lousi said. "Hopefully, I can help them push forward and achieve further success over the next few years."
In another example of Kiwi coaches signing their own, Lousi was lured by Crusaders assistant Brad Mooar who takes the reins at Scarlets from Wayne Pivac at the end of this campaign.
"I tried to sign Sam last year but he was still under contract," Pivac said.
"He's a fine player and I think he will do well. Brad knows him well from coaching against him. He will suit the way the Scarlets want to play the game – a big man in the middle of the park that can offload.
"He's got a rugby league background. We have seen forwards like Sonny Bill Williams come out of rugby league and have the ability to offload in traffic, which is a big plus against the defences we face these days. You can never be too physical so the more big men you have got who are mobile and have got great body positions the better."