What skill tier are you? Because it makes a difference. Also I take it you're a roamer. This is Tier 6 advice, not Vainglorious, so take it for what it is.
Protecting teammates only really helps if the teammate is useful to keep alive. So if they're building stupidly like.. I don't know... bone saw first item on koshka, or 3 defense items on a ringo then don't even bother. You'll get a lot of this around new patches or on the weekends, especially in lower to mid tiers. You're better off keeping yourself alive, farming up a few damage items yourself, and trying to semi-carry.
If they've got a decent build but have poor positioning or got ganked/shut down early and tilted a bit, then it's a different story.
Keeping them alive is a combination of items and positioning, and may involve you dying instead of them.
I find my favourite roams to be:
Cath. Just be sure to stun a lot to build up her passive. max bubble and ult. Get a clockwork fairly early to stun more.
Glaive. Get warhorn, fountain, clockwork, aftershock, t2 boots, and one other item your choice. I like double clockwork because it's like playing tennis against yourself with the enemy team.
Celeste. Don't do this in ranked until you practice. She's hard to get used to but so powerful as a roam because she's got an AOE stun on a 3 second cooldown that is ranged. She's the only roam that doesn't have to run all the way over to the carry or enemy to save them. also she hits hard. Get warhorn, t2 boots, clockwork, fountain, aftershock, and either crucible, atlas, or broken myth depending on how the game's going. Always ban taka if you try this out, he's you're kryptonite. I like her roam because with other heroes you dive in to save teammates and they think you're engaging and turn around to die. With Celeste, you save them from a distance so they keep running.
Koshka. She can dominate the enemy early game, and by level 6 as a roam she's got a handy disable. Probably when you use it though, the carry will turn around to re-engage with could cause their death, so she's hit or miss as a roam.
Honourable mention:
Heroes I don't like for saving people:
Ardan. I never play him, but he has that vanguard thing. I guess he's good. You can play him and save people. I just haven't, that's all.
Phinn. He's too slow. He has his ult, but more often than not you'll drag the enemy toward the carry you're trying to save. Also he's countered by krul.
Petal. Once upon a time she used to roam. No more. She has that heal but needs crystal to use it so she's better as a jungler.
Adagio. He has that heal but he needs crystal items to really save people, so he's better in lane right now IMO.
Fortress. He's like an attack support. No stun on him.
Ozo and Joule. You have the ability to roam and a stun to save people but I haven't been able to pull it off well because you end up in a bad position so often that you end up feeding.