Mordin
Role:
Mordin (provided he survived ME2) helps create a cure for the genophage, securing the krogan clans for the effort to liberate Palaven. He is a spokesman for the STG during the war summits, a top scientist who helps oversee the construction of the Crucible, and creation of special anti-husk weapons for the rachni.
Recruitment:
After Shepard returns to the Normandy after the first war summit, he gets a call from an STG agent giving details on where to locate Sur'kesh's STG base with sensitive information regarding the possibility for a genophage cure.
If Mordin died in ME2, then Shepard/EDI still has records of the genophage data that Maelon tried to create six months ago. STG wishes for Shepard to bring the data with him to the planet.
If Mordin survived ME2, then the STG agent will explain to Shepard that “someone” Shepard knows needs help moving genophage data safely off of Sur'kesh, and this someone suggested recruiting the Normandy SR2 for this task directly.
Shepard can select the planet Sur'kesh on the galaxy map to investigate the STG agent's claims.
Once Shepard lands on Sur'kesh's STG base, he is met by security officers who take him down an elevator into the heart of the base. As they pass by rooms filled with STG projects, Shepard sees a number of strange things; a sleeping yahg being dissected in an operation room – apparently implanted with some sort of tracking device; STG operatives performing weapons tests with unique guns; and a plethora of other questionable experiments.
If Grunt was not recruited in ME2 (via modding), he will be found encased in a Okeer's incubator. The STG operative explains that Okeer was killed while perfecting this “legacy”, and a mercenary named Jedore sold the genetically perfect krogan to the salarian government for a billion credits.
If Grunt was left in his tank in ME2, then it is revealed that, after the suicide mission, the Alliance/Cerberus sold Grunt to STG for a billion credits.
-furthermore-
If Rana Thanoptis survived ME1 and ME2, she will be working for STG, and Shepard will find her at Grunt's tank, studying his life signs. Rana says that by studying the krogan.
If Rana survived ME1 and ME2, but Shepard either killed, or did not recruit Grunt, Rana is not located at the STG base.
Shepard is lead into a massive vault containing a small vial of green liquid suspended in a energy field. When Shepard asks what the vial contains, Mordin enters the vault and replies to Shepard's question, saying that the vial can effectively secure an alliance between the krogan and council races. After greeting Shepard, Mordin explains that the vial contains the partial cure for the genophage.
Shepard asks Mordin what they need to do with the genophage cure, and Mordin says that it needs to be moved to another location: Tuchanka. Mordin is actually Wrex's/Bakara's mole in STG, updating them on the progress of the genophage, and can take the vial to clan Urdnot's territory where it can be replicated by Bakara's scientists, and distributed to the population.
Shepard asks Mordin how he managed to create a cure so fast, and Mordin explains that, as soon as the mission with the Collectors was over, he knew the Reapers were mere months from invading. Mordin gathered the best STG minds to start formulating another cure.
If Maelon was not killed in ME2, then he will be working with Mordin again – going through what Mordin calls “the right process of scientific experimentation”. Maelon still believes that he was helping the krogan more than he was hurting them. However, his data, if recovered in ME2, sped up the process of coming up with a cure greatly.
Ultimately the group came up with an altered genophage cure much like the initial one they worked on which curbed fertility rates, so that 1 in 1000 births would produce living offspring. The new cure was much more effective, so that roughly 1 in 100 births would produce living offspring. Mordin is pleased with this.
If Mordin kept Maelon's data in ME2, the the genophage is completely cured. STG kept this information from going public, because Mordin feared that the salarian Dalatrass would confiscate the cure (she is renown for her support of the genophage).
Mordin puts the cure vial in a large crate, and puts the crate onto a loading dock to be taken to Shepard's shuttle. Suddenly alarms start going off, and the base is rocked by a massive explosion. Salarian soldiers wearing black and green armor crash through the base and start attacking the STG operatives and scientists. Mordin theorizes that these are actually troops of the Dalatrass, and that she must have a mole in STG as well (something STG half suspected).
Shepard and Mordin fight their way though the base while protecting the crate containing the genophage cure. As the action climaxes, one of the Dalatrass's operatives shuts off power in the entire base in an attempt to stop the loading dock from moving. As a result, the yahg in the operating room manages to wake up, as it is no longer being sedated. It goes on a rampage, destroying large sections of the base as it tries to escape, and finally spots Shepard's shuttle. Shepard and Mordin (who have not yet reached the shuttle) try to distract it before it can claw its way inside the kodiak – but this only enrages the giant alien causing it to bullrush them instead.
If Shepard did not recruit/unlock Grunt in ME2, the krogan super soldier, unleashed from his cage in the power shortage, bursts through the floor, spots the yahg, and attacks it. The two aliens fight brutally, while Shepard and company abandon the base, taking the genophage cure with them. The last Shepard sees of the two giant aliens, is Grunt wrestling the yahg off a balcony to an ambiguous fate.
If Rana survived ME1 and ME2, and Grunt was not unlocked in ME2, then the former's tampering with the genetically pure krogan grant him even more combat skill. Grunt kills the yahg by wrestling it onto its stomach, and snapping its neck while Shepard and company load the genophage crate onto the shuttle. Shepard boards the shuttle, and sees Grunt stalking towards him. The krogan is distracted by gunfire from some of the Dalatrass's soldiers who just entered the docking station, and turns on them. The last Shepard sees of Grunt, is the krogan picking up an assault rifle for the first time, grinning at the mass effect rounds bouncing off his kinetic barrier, and charging the Dalatrass's men to an ambiguous fate.
The genophage cure is brought to the Normandy, and Mordin is pleased to be back onboard, starts working with Dr. Chakwas and/or Dr. Michel on ways to come up with a mass distribution method. Mordin is automatically taken to Tuchanka.
Missions:
Mordin is a temporary squad-mate for the mission “Sur'kesh: STG Base”. After the completion of this mission, Mordin goes to Tuchanka's hub world where he assists Wrex/Wreav in formulating distribution for the genophage. He is an available temporary squad-mate for all missions on Tuchanka – including the main mission “The Cure”. Once the genophage story arc is completed (provided Mordin survives), Mordin goes to the Citadel to assist the war effort more directly. He is available for the missions “Hunt a Killer”, “Track a Killer”, “The Hidden Wards”, and the final assault on the Reapers.
At the finally of the mission “The Cure”, Mordin and the salarian Dalatrass (and Maelon, if left alive) argue over the ethics of curing the genophage.
Maelon's perspective represents the paragon option for the cure: a complete cure of the genophage.
Mordin's perspective represents the paragon option for the cure: a partial cure for the genophage.
Dalatrass's perspective represents the renegade option for the cure: not administrating the cure at all.
Potential death:
Mordin's death is completely dependent on the ultimate outcome of the genophage.
If Shepard agrees to follow through with Mordin's perspective on the cure, krogan dump the cure into massive water pump system that goes throughout the main continent of Tuchanka. At that moment, a small Reaper bombards Urdnot camp, damaging the subterranean tubes from which water flow from the pump system. Someone must go into the pump system, activate a security override, and pump water into alternative pathways, or the cure will be drained into Tuchanka's radioactive soil. As the Reaper descends onto the site, Mordin cheerfully decides to go into the pumping system to perform the override, while Shepard and Urdnot clan face off against the Reaper. After the Reaper is defeated, Mordin opens up all alternative routes for water flow, causing the very area he is standing it to be flooded. As a cascade of water rushes towards him, Mordin hums his rendition of the “Gilbert and Patter” song.
Personality:
Mordin's personality is largely unchanged since ME2. He still values a modified genophage over a fully cured genophage – and he still is as energetic and full of ideas as always. Mordin gets along well with all of the crew, especially Samantha Traynor and Liara.
Mordin plays chess with Samantha three times over the course of ME3. The first two times, the two come to a stale mate. On the third time however, Samantha manages to beat Mordin – much to his surprise. Fascinated by her intelligence and tactical prowess, Mordin chats with her often over the intercom.
Mordin likes to give Liara helpful advice on where to hide bugs, plant agents, and extort evidence. While Mordin is slightly more ruthless than Liara is comfortable with, she never the less is appreciative of the experiences that STG taught him.
Ironing out variables:
If Mordin was killed in ME2, he does not appear in ME3. He is replaced by an ex co-worker named Wiks. Wiks believes that the genophage should be cured partially too – though he is not against the idea of completely curing the genophage.
If Mordin's loyalty quest was not completed in ME2, Mordin has no access to Maelon's genophage data. Mordin comes up with a crude but effective cure in a period of months (working from scratch), that fertilizes 1 in roughly 400 krogan eggs.