As soon as Ren set foot on the island the first thing he did was to use his spiritual sense to check his surroundings, and notice the figure of the masked Haku hidden nearby spying on them, so Ren decide to call him out "Heeey Haku, come out we have come to help you as you have requested"
Then Haku comes out of his hidden place and says "Just the two of you? I thought that you would have brought at least Kakashi and his team with you..." and Ren replies "Well think about it, I'm sure that your possible allies would be a little worried if some Konoha's ninja, especially one as famous as Kakashi-san, come with you at the meeting, and as you can see we are not affiliated with any village and I can assure you that we are strong enough to help you..."
Before Haku could say anything a terrible blood lust come from behind him and started putting pressure on Ren, Peri and Dosu, Ren was able to remain calm even under this pressure, while Peri was trembling and Dosu put up his guard and was ready for anything.
Soon from behind Haku the figure of Zabuza could be seen and as he looked at their reaction he said "Heh... at least you can still move under my pressure so you aren't totally useless but how can we be sure that you can help? And more important why would you help us with this thing? I know that in this world nothing is free"
Then Ren looked at Zabuza in the eyes and said "What I want from both of you for my help is simple, I want the two of you to join in this little group that I'm creating who main objective is to protect as many people as possible while we hunt and put down the most merciless and evil people we can find right now on the list are Yagura, and if what Haku told me is true the one behind him, that transformed Kiri in the Blood Mist, Orochimaru a crazy ninja that use civilians and his own people as a test subject, and Danzo, many of you do not know him but he is a comrade of the late third Hokage, but unlike him he would do everything for his own benefit, I would not be surprised if he was the one who helped Orochimaru with his attack on Konoha"
Then he continue "And for how can we help you why don't we spar a little so you can find just how strong we are?", Zabuza stood in silence for a few seconds the he unsheathed Kubikiribōchō and says "Very well, let's see how strong you are, as for joining your little group, I'll think about it if you beat me"
Just as Zabuza was about to attack Haku interrupt him "Wait Zabuza-san let me go first..." Zabuza look at Haku and the say "Okay", while Ren seeing this turn to Dosu and says "He is yours, I'll fight the strongest one, but be careful that guy is fast and can use Water, Wind and Ice Release"
Dosu clicks his tounge then says "Okay...I'll fight" then while Ren and Zabuza retreat, Dosu and Haku move to the closer one other, then without warning Haku with one hand throws some senbon to Dosu, while with the other he does one hand seals.
Meanwhile Dosu deflect the senbon with his gauntlet then rush at Haku who just finished doing the hands seals and directly use the Makyō Hyōshō (Demonic Mirroring Ice Crystals), and creates several large mirrors out of ice, which he surrounds Dosu with, then by entering one of these mirrors, Haku's reflection appeared in all of them.
And soon Haku starts moving from one mirror to another while throwing senbon at Dosu, who while he deflect some of them with his gauntlet the others pass by his defence and hits him in various places, seeing that he can't continue defending Dosu raise both his arm and then slams them into the ground while saying "Kyōmeionpa (Resonating Echo Wave)" and generating a strong sound wave that crushed anything around Dosu.
Haku noticing that he wasn't safe got out of the mirror just before it was crushed in pieces and distanced himself, meanwhile Dosu took the opportunity to get close to him and started swinging his arm in a wide arc, but Haku evade the blow with minimal movement or so he tought , as after he evade Dosu swing he started feeling dizzy and nauseous, that was in fact another Kyōmeisen (Resonating Echo Drill).
So Haku decide to once again distance himself and went where the remaining crushed pieces of the ice mirror were and after doing some fast hands seal he says "Hyōton: Tsubame Fubuki (Ice Release: Swallow Snow Storm)" and creates a cluster of ice needles in the shape of miniature swallows out of preexisting ice, which then launches at Dosu.
These needles moves in the direction of Dosu and he tries to evade them but the still keep following him , and when he hides behind a tree, those needle are so sharp tha cut it in many places, so having no other option Dosu once again use the Kyōmeionpa only this time he claps his hands in the direction of the needle that are soon crushed in many small pieces.
But the sound wave continue in the direction of Haku who does another set of hand seals and then says "Hyōton: Hyōgan Dōmu(Ice Release: Ice Rock Dome of Magnificent Nothingness)" and quickly freeze the surrounding air and create an icy, transparent igloo-like dome around himself to act as a defence.
So the sound wave and Haku's ice dome clash resulting in a huge dust cloud, meanwhile Dosu his panting hard, as the last jutsu took out a huge chuck of his chakra, but just as Dosu was about to confirm Haku's state, he felt one senbon near his throat then he heard Haku voice that said "This one is my win, you are almost out of chakra, but I have to say that if you had a better control of your chakra, then I'd be in trouble"
Then Dosu and Haku distance themself and as Dosu come close Ren says "Good job, do not worry about losing, he just got more experience then you, and he is better at not killing his opponent, i know that you have another more lethal jutsu, thanks for not using it, by the way what do you think about our future teammates?", and Dosu replies "They are strong for sure, I felt that he was also holding back a lot, all my injuries are in not lethal place and can be healed in a day or two, but are you sure that you can convince them, that Zabuza guy it isn't as gentle as Haku"
Then Ren faking wiping his tears away says "I'm moved, you are worried about my well begin...." and Dosu interrupt "Shut up asshole, I'm grateful for you helping me, after all if it wasn't for you I would be another of Orochimaru test or even already dead as a sacrifice or something like that, this is also why I chose to join your group other than the revenge reason", and Ren says "Well, do not worry after all I'm stronger than you think" then he put down Peri and moved toward the battlefield where Zabuza was already waiting and said in a low voice "It's time to stop holding back at every occasion"