To me Io is a champion not meant for constant backline work. As a healer she has great mobility with Lunar Leap that can be used in more situations then running, so I prefer a build that allows me to move around the map a bit more confidently with that in mind. As I know I can move Luna to me rather quickly with Lvl 3 Spirit Arrows if I need some thing to heal to regain health with my Lvl 3 Restored Faith, or to use Luna as a shield to regain health and dodge damage. I know I will always have a chance to retreat because of Lvl 2 Crescent which has saved me many times and allows me to be more confident when I do try and push up to heal certain characters that may be out of my range had I stayed in the backline. Speaking of range, I have Lvl 2 Broken Deity with Life Link, that allows me to place Luna in some hard to reach places were an ally could be hurt that I couldn't normally reach otherwise. It also allows me to place Luna on point much quicker and combined with Spirit Arrows, allows me to move Luna around fast enough to heal her if she's damaged or to get her back in action to heal others. Then I have Lvl 5 Moonlight Garden, because if I'm not moving around the map healing then I'm shooting, and it gets my Moonlight up the fastest and most efficiently. Though it is a double edge sword if you aren't confident in aiming or just happen to miss your shots. Her cards and my game knowledge also allow for me to take on a flank in certain occasions, or stall a flank long enough to get help or get out, because Luna becomes more Mobile from Spirit Arrows it makes it easier to move her to me if I don't have a teammate to heal to heal myself from restored faith, and it allows me to use her like a shield as I said earlier, because when Luna is close to you, she becomes a shield for most straight forward attacks. That is my Io build! Hope you like it and try it out for yourself. Of course this build fits my playstyle with Io so I know for some people the loadout may seem odd or certain items unnecessary, but having played Io for as long as I have I know I work best with these items.
Increase Luna's deploy range by 40%.
Reset the Cooldown of Lunar Leap after dropping to or below 30% Health.
Reduce the Cooldown of Luna's redeploy by 0.30000000000000004s after healing with Moonlight.
Heal for 30 every time you Heal an ally with Moonlight.
Generate 10% Moonlight for each enemy hit with Light Bow.
Your weapon shots reduce the effect of healing on your target by 25% for 1.5s.
Increase your Ultimate charge rate by 10%.
Reduce the Damage you take from Direct attacks by 7%.
Increase your Mount Speed by 15%.
[Guardian Spirit] Luna Heals allies around her for 200 per second.