Loki is actually the warframe I started the game with! I had no idea how to use him properly at the start, but once I figured him out he was a powerhouse! Radial Disarm was what allowed me and my friend to go well beyond what out gear should allow us to handle in Tier 3 Void Keys! It's funny how far the mighty has fallen, with Loki nowadays being relegated to Switch Teleporting moving objectives to get them to their destination faster as his only job.
Invisibility and Radial Disarm are still really good abilities, just that other frames can do both things better (cough Octavia cough). Switch Teleport has it's niche gimmick but outside of it, it's completely useless. And Decoy really ought to not have limited hitpoints, it's a holdover from ye old days that should have been removed long ago.

Loki does strike me as bi, yes.

Loki comes painfully late on the star chart, itd be nice to new players to have him on Mercury. Since Ivara obsoletes him for stealth, but only comes from grinding spy vaults until you're blue in the face, it'd be helpful to actually have a spy frame available before you've nearly completed the star chart.

As far as his kit goes, he is woefully non-competitive. He can move around the bounty drones and skip a few spy vault obstacles but neither is particularly necessary. You could just be a volt and rush both with sheer speed and have more usable abilities.

I dont know what to do with Decoy other than set up a teleport. Just use invisibility if you dont want to get shot.

Radal Disarm doesnt play nice with lokis other desired stats, And you can just use invisibility if you dont want to be shot.

I’m apparently in a small club of people who think that once you can helminth abilities, Loki is great.

I think he’s arguably tied for best generalist in the game. He can do any mission type, though Switch Teleport was SO good at Disruption that DE nerfed it in the first week and now that mode is his weakest. Still, when I don’t know what the mission will be (which means any new quest) I take Loki, because I know he will be fine. Maybe not optimal, but optimal is boring much of the time. Loki completely punks the Protea specter in the Deadlock Protocol, for example.

With Golden Instinct on Loki, he became my go-to frame for Zariman farming. Armed with the Ratka Cernos, I use him for probably 85% of my Void Angel farming. Angels have no way to handle invisibility aside from that red AoE mist, which Ivara is too slow to get out of. Loki manages it just fine.

Savage Silence Loki is the low range Finisher murder fun that used to be possible with Banshee, pre-Eximus rework.

I also disagree that Ivara makes Loki unnecessary for spy. Yes, Ivara trivializes spy—and exterminate, and capture, and survival—but she’s very slow. When you’re trying to get things done quickly, Loki is your spymaster!

Loki is a bit of a niche warframe, whose main use is spy but also provides utility elsewhere outside of spy vaults.

Decoy - This ability actually synergizes extremely well with Switch Teleport, letting you bypass lasers by simply placing the decoy on the opposite side and switching places with it, although I rarely see anyone playing Loki making use of it inside spy missions.

Invisibility - A pretty basic ability, but as long as enemies can't see you, you're pretty much invulnerable. Enemies do try to "guess" where you are by firing their weapons towards where they were getting shot from, and you'll have to watch out for AOE attacks. If you don't like the orange filter when you go invisible, you can disable it by going to Settings > Video > Color: Off.

Switch Teleport - This is probably more of a gimmicky ability but is very useful for certain objectives, especially when combined with the Safeguard Switch Augment. My main use for this outside of Spy is escorting drones in open world missions thanks to how far it can swap if you're modded for range, and Safeguard Switch means that you don't need to worry about it being teleported into a group of enemies. Use either a Sniper or Mining Drill set to high zoom to make targeting the drone easier at a distance. My other use for this ability is Gas City fissure missions to help teammates that are struggling to parkour across the large gaps.

Radial Disarm and Irradiating Disarm (augment) - This is a fairly strong CC ability, forcing enemies to melee instead of being able to use their guns, and with Irradiating Disarm, enemies will tend to prioritize fighting themselves instead of trying to attack an objective. The augment is probably most useful for Defense/Excavation but may be a bit of a hinderance in Survival because they will be focused on fighting themselves instead of moving towards a centralized point around the player. This ability does suffer from the update that add Overguard unfortunately, and this is very noticeable when playing a mission with increased Eximus units as a modifier.