Updated, revised version of my original Yareli build.

*Note: this is actually a 4 forma build because you can still swap polarities with the aura slot. If that ever gets changed this goes back to needing 5 forma.

This no-helminth build is intended to buff your secondary weapon damage as much as possible, increasing its base damage, crit chance, fire rate, and reload speed, while giving over a 300% damage multiplier from constantly spamming sea snares. And yes, this does require you to be riding Merulina. [Yareli] benefits so much from it it's a waste to use [Loyal Merulina].

This build utilizes a rank 0 [Combat Discipline] to minimize the self damage to Merulina and completely eliminate the damage to [Yareli], since any damage instance <0.5 will get rounded down to 0, and the self damage is affected by Merulina's damage reduction. Because Merulina is still taking some damage, this allows you to consistently make use of [Archon Intensify], since killing enemies affected by sea snares will heal Merulina with [Merulina Guardian], which triggers the effect. Healing Merulina also triggers [Merulina Guardian]'s other effect, giving you 200% fire rate and reload speed to your secondary weapon. You should be able to reliably refresh the duration of the augment whenever you need it. Finally, the self damage will also constantly proc Arcane Avenger, giving you an automatic 45% final crit chance to any weapon you use. Combat discipline allows you to have 100% uptime on all of these benefits.

Stats-wise, [Yareli] works very well without mods, so most of these numbers are up to preference.

IMO, the most important stat is efficiency only because so many different interactions in this game do not work while [Yareli] is riding Merulina. Having high efficiency lets you dismount and remount at will without tanking your energy, especially at the start of a mission before you've filled your energy pool. 315 base energy is enough for anything she needs to do, but the high efficiency helps in many less common situations. If you find this is overkill, replacing [Fleeting Expertise] with [Streamline] is fine.

Strength is at a comfortable 200% (230% after [Archon Intensify]), giving a 330% damage vulnerability multiplier to enemies affected by sea snares. This also starts you at 15,000 (17,250) health for Merulina, which is plenty for most content.

Duration is not as necessary as you would think, since it really only matters for the duration of [Merulina Guardian], which you should already be refreshing by constantly casting sea snares. Aquablades has a base duration of 45 seconds, so 8% less is not an issue. Because of this, I only tried to keep it close to neutral, mostly for the sake of comfort. I am using [Augur Message] over [Constitution] for some rare occasions where having overshields is necessary, such as with certain weapon incarnon perks. The negative duration also has a niche benefit of making Riptide end earlier without you having to think about it.

Positive range is good for Riptide's grouping capability. It's base range is okay at 15 m, so a bit more is nice. I have [Archon Stretch] slotted in for this, only because I don't mind the extra investment, so why not. [Archon Stretch] is activated by my companion modded to deal electricity damage, so it should be fairly consistent. It should be noted that [Archon Stretch] doesn't actually work while [Yareli] is riding Merulina, so I would consider it only useful in very rare cases where you are dismounted and have completely run out of energy (update: this got fixed, so it actually works now while riding Merulina). A single proc of [Archon Stretch] will generate enough energy to cast Merulina once. Regular [Stretch] is more than fine if you don't care for that. You might be able to save a forma in that case.

In terms of survivability, [Yareli] is one of the most survivable frames in the game. Having innate 90% damage reduction allows her to easily tank enemy damage up to around level 500 with [Adaptation]. Above that, she starts relying on her much more effective survivability tools. Specifically, casting Merulina gives her 4 seconds of invulnerability while absorbing any incoming damage into Merulina's hp. Whenever Merulina dies or [Yareli] dismounts, she receives another 1.5 seconds of invulnerability, which she can use to remount Merulina. This means that [Yareli] can be effectively invincible at all times, even up to level cap. It is important to note that once enemy damage gets high enough, your shields will start breaking without killing Merulina, meaning that your health will be left vulnerable if you don't get knocked off. For that reason, the best strategy is to dismount and remount whenever your shield breaks. Between the i-frames from remounting, shield gate, and dismounting, your shields should be able to recharge each time you do this. If your goal is higher level (>1000) content, you are probably better off replacing [Adaptation] with [Fast Deflection], just to improve your shield gating capabilities. This strategy is basically shield gate+ with the multiple sources of i-frames you have access to. The modded 160% efficiency also makes this significantly more economical than traditional forms of shield gating.

For archon shards, use whatever you want. I have 5 Tauforged Topaz for an additional 375% crit chance, just because I like the red numbers. It also allows me to free up a mod slot on most weapons.

As mentioned before, arcane avenger is here to further step up those crits. Arcane precision is an easy 300% base damage increase, so it's a pretty obvious choice. If you find ammo economy to be lacking, or fire rate (hard to believe, I know), you can swap either of these with arcane pistoleer or arcane velocity.

In terms of loadout, make sure to have your [Furax] equipped with [Amalgam Furax Body Count] for the extra 45% fire rate. I also run [Yareli] with my beloved Panzer Vulpaphyla for an additional 60% fire rate and 1.2x crit multiplier, as well as the viral spread. This panzer build is a bit outdated, however, since the most recent companion rework hasn't been added to Overframe yet.

For weapon choices, Sporelacer is my go to for most missions. I also love using my Ocucor, Dual Toxocyst, Coda Catabolyst, and, surprisingly, Kompressa prime. [Merulina Guardian] makes secondaries super ammo hungry, so I make sure to have some sort of ammo solution for all of them.

[Yareli] is my favorite warframe thank you for coming to my TED talk.