- "His power is growing with the virus... It's too dangerous to keep them loose! Please... at least do something about Zero... before it's too late!"
- ―Lifesaver, Mega Man X5
Lifesaver (ライフセーバー Raifusēbā) is a Reploid who works as the Maverick Hunters' medical control officer in Mega Man X5. He runs virus tests and builds vaccines to protect against them, and is very knowledgeable of the Sigma Virus and its properties. He's also responsible for general medical duties, such as when he picked up the exhausted Zero after the Shuttle mission. Lifesaver was always suspicious of Zero's virus readings, as the virus seemed to have no effect on him. When the Zero Virus broke loose, Lifesaver's suspicion of Zero grew even greater. When X confronted Zero about his condition, Lifesaver, who had accompanied X, immediately accuses Zero of being infected.
Some scenes seem to indicate that there is more than one Lifesaver, suggesting that all Lifesavers are from the same mass-produced model. While this may be the case, in the events following the crash of the Eurasia, they/he made no further appearances with the Hunters during the Nightmare Outbreak, nor during the war between the Maverick Hunters and Red Alert or the incident with the Jakob Project.
Lifesaver is, in general, a very cautious being. He really can't be blamed for that, having such extensive experience with the Sigma Virus. When he shared Zero's virus data with Signas, he recommended something be done immediately, and was displeased with Signas' refusal to take immediate action. He jumped to conclusions with the Zero Virus confrontation, but it seems fair to do so, given the circumstances of what he was faced with.
Lifesaver discussing Zero's condition with Signas
Lifesaver: ...That's all of the report on Zero.
Signas: Zero has fought Sigma several times, I believe he has the Virus antibodies.
Lifesaver: It's possible... He doesn't react negatively to the Virus... His power-output is actually growing... For the most part, the body systems of X and Zero remain mysterious. I can't predict what will happen... I suggest you take measures...
Signas: Keep this secret... I'll think it over.
Lifesaver: His power is growing with the virus... It's too dangerous to keep them loose! Please... at least do something about Zero... before it's too late!
- We've found Zero. He's safe but he's very exhausted. We'll return to the Hunter Base now... (A Lifesaver pulling Zero from the shuttle escape pod.)
- Look...! Just as we expected...The Virus is everywhere, and yet he is not infected at all. He must've already become a Maverick! (Three Lifesaver's (along with X) confront Zero (when playing as Zero in the final area).)
"Lifesaver's got a super-slick design. But he shows up even less than Douglas. (sweat) He does touch on the core of the story, though, so go easy on me. If he seems strong for the role of a doctor, perhaps it's because he was originally planned to show up as an enemy? There was no set-up for him as an enemy originally, but if I could have them [you?] play, maybe they'd realize, 'Oh! I wish they'd established him like that!' (Nah, I don't think they would (grin), but this is revealing a bit too much of the idea source, so I can't say any more.)" 
"This was the first of the sub-characters I designed. Pay attention and you'll see he has a more mechanical feel to him than do all the other Repliroids. I thought there was only one of him, so I was a bit surprised that a whole group shows up. Somehow it seems there are at least more than three of him/them." -Haruki Suetsugu 
"There are many Lifesavers, and they feel less 'human' than the others, so I tried to convey this by reducing the amount of fleshy body parts on them." -Haruki Suetsugu, Mega Man X Official Complete Works, 2009