Etranger is a bizarre clan. It has no signature abilities, and is loaded with clone cards (sometimes even having clones of things it already has). Despite all of this, Etranger is lacking in one essential area: it contains exactly four units with "boost". This includes the grade zeros, which are neither starters nor triggers. As a result, Etranger used to be completely unusable. Until this came out.
The skill of Princess of the Corals, Tethys allows her to gain +1k for the turn at the cost of CB1. As an ACT skill, this can be used multiple times per turn if need be. This isn't what makes Tethys important. Tethys is the most important card for an Etranger deck because of her second skill, which treats her as Aqua Force. The only other cards that can be treated as another clan are the Blaster Spirit cards. It's an interesting feature, but it's nothing special in developed clans. Etranger is notably not developed. However, if an Etranger deck were to run Tethys, along with other Aqua Force grade zero and one units, it can use trigger effects. Remember, in order to use triggers, you must control at least one unit from the same clan. As Aqua Force, Tethys opens up an entire clan's trigger library, boosting units, and starting vanguard (the things Etranger are most lacking).
Strategies and game play:
Tethys is always an Aqua Force, even if you ride her. This means that you can use Forerunner skills from Aqua Force. Aqua Breath Dracokid is recommended, because most of the cards on the backrow of an Etranger/AquaForce deck will be Aqua Force (there is only one exception, Queen Lyne, who has the same skill as Tethys but is not a Dual Clan) and most of the cards in the front row will be Etranger (so skills won't work after you ride to grade two). Other than that, Tethys is best sitting safely in the back row where she can enable triggers and not get attacked.
Combos with other cards:
As a Dual Clan, Tethys can use the forerunner skills from two different clans. Etranger has none, but Aqua Force has a lot. A complete list: Advance Party Brave Shooter, Aqua Breath Dracokid, Battle Siren Carri, Officer Cadet Erikk, Officer Cadet Astraea, and Tri-Holl Dracokid. In a mixed Etranger/AquaForce deck, Etrangers need to be the ones in the front row (because they have attackers in abundance compared to their amount of boosters). Therefore, "if you have an <<Aqua Force>> vanguard" skills won't work after you get to grade two. Front-row Aqua Force units will be rare (if they're even used) in a mixed build, so skills involving boosting Aqua Force is not recommended, nor is the grade three searcher. That leaves Aqua Breath Dracokid, who has a skill that can give one other Aqua Force +1k for a turn at the cost of putting himself into the soul. No vanguard requirements on when he can do that. If you're 1k short of the "math trick" (attacking at exactly enough power to force an extra guard), you can give the booster for that column more power and clear up some space to play a stronger booster.
Ways to counteract it:
Kimnara and Blazing Flare Dragon. Goku too to a lesser extent. Goku specifically retires from the backrow, but at random times. Kimnara can CB1 and soulfill himself to retire a booster. Then, when Blazing Flare gets to five soul, it can SB5 to retire anything. Target the backrow with retire effects and you can prevent the opponent from getting trigger benefits for the turn. If you can also force them to use all of their Aqua Force from the hand to stop attacks, they will either need to draw into a new Aqua Force or accept that, until their next turn, they can't use triggers.
Makes Etrangers usable by giving them triggers, boosters, and a starter.
Triggers can't do anything without the same clan, and eliminating all members of that clan from the field will be fatal.
Tethys is a promo-exclusive card with only one art. Apparently she was originally from a card game called Monster Collection (MonColle), but I have yet to see any cards of her from that game.
Artwork and aesthetics:
She's a mermaid. That's it.
She has an average skill, the kind that would be replaced by nearly anything better (2), but she also is a Dual Clan (+1) that makes a clan able to use starters and triggers (+2) at the cost of making nearly half the deck a different clan (at which point it may be easier to just run the other clan in a Mono build) (-1). While still a really good card in her place, that place is so arbitrarily specific that she only scores 4 out of 10 overall.