I don't know what you consider endgame, in my opinion she is amazing in arena defense. Nearly unkillable in certain situations. 

She doesn't really bring any value on defense, though. Her passive is okay, but heavily eclipsed by champs like Scyl, Doompriest, and Tuhanarak. Since her kit adds nothing to your offense, she'd probably be paired with other defensive champs like Seeker/Vogoth, but then you have to ask - where's the spot for her on that team?

A "normal" go-second team involves champs like Seeker, Vogoth, Lydia, Rhaz, Scyl, Mortu, etc... Most of those bring much more value passively than Wythir does.

Not for a go 2nd attack team. But for a tanky defense set up.  She is prevalent in gold tag defenses and high G5.  Paired with siphi, duchess, nekhret, mith, cardiel, ... some combo of those.  And 1 attacker like candy, trunda, Leo, rotos w/siphi

Or can be built to solo/duo IG and dragon 

I am seeing her in G5 at reset right now. Slots right into these tanky tanky Duchess teams that look to survive and just SMACK YOU when they finally go.

amazing cleanser and healer, i use her in doom tower.  replacement for reliquary tender for me.  pair her with rector and vogoth in arena and i think they'd stall out a lot of teams, or any similiar tanks.

Yah I saw a screenshot of plat tier teams, and it does seem like she is popular. I think I'm just missing the requisite pieces. 

My current go-second team is [Lydia/Rhaz] / Raglin / Vogoth / Mortu. Waiting to pull a Seeker to put as lead. At that point, I guess the question would be - does Wythir have more purpose on the team than Raglin?

If you are endgame, I can really only see in arena for her. G4-5, maybe 

It seems so. She is much more of a pain to deal with, in my experience.

The  way  I  view  Wythir  is  as  a  PURE  Healer  with  side  bonuses

Wythir  does  PURE  Healing.

Wythir  A3  -------------->  Heals  based  on  her  Max  HP

Wythir  A2  +  Passive  --->  Heals  over  time  with  Continuous  Heals

Wythir  A1  --------------->  Heals  through  combat  by  providing a  Leech  Debuff 

If  you  think about  it,  Wythir  is  healing  in  almost  every  possible way  a  hero  can  be  healed.

It's a  little scary to  think about  because not  many  heroes heal  as  much.

I  think it  makes  Wythir  pretty unique as  individual champion.

Wythir  does  Increase DEF,  but  it  isn't super  amazing like  Seekers.

Wythir  does  Remove  Debuffs,  but  it  isn't super  amazing like  Tuhanaraks.

I  came  to  the  Conclusion that  Wythir  main  role  is  as  a  Healer.

Wythir  Increase  Def  &  Cleansing  are  not  super  amazing,  but  they  are  still  nice  secondary  things  to  have  which I  would consider to  be side bonuses.

In  my  eyes,  Wythir  can  be  amazing on  a  Go  2nd  team.

I  made charts  to  showcase what  I  think  is  Normal.

i

Everyone has  different opinions  on  what  they  think  Normal  is  which  is  fine.

i

I  think  Wythir  could be  used  on  a  Classical  Resist  team  as  the  2nd  to  move  Champ.

A  team  I  could envision  is  the  following:

1st  move  -  Seeker

2nd  move  -  Wythir

3rd  move  -  Wurlim

4th  move  -  Rhazin

Seeker  in  Retalition/Avenging  set

Wythir  in  Shield  set

Wurlim  in  Resistance  set

Rhazin  in  Resistance  set

Obviously,  The  set  up  I  am  suggesting is  different  vs.  the  team  you  are  recommending.

The  issue  is  I  feel  like  the  team  you  are  talking about  is  not  a  normal  team.

I  think  you  are  creating  a  non-standard  specialized  team  designed around  Mortu.

The  set  up  I  made  with  my  Mortu  is  a  Specialized  team.

The  set  up  is  designed  to  Counter  Blender  teams.

i

You  have  to  remember Mortu  skills  are  weird.

On  Go  2nd  team,  You  are  focusing on  HP  +  Mortu  Secret  Skill  +  Ignoring ATK

On  Go  1st  team,  You  are  focusing  on  ATK  +  Mortu  A2  Skill  +  Ignoring  HP

Now  lets  assume  you  want  Mortu  on  Go  2nd  team.

The  question you  have  to  ask  yourself is  how  does  Mortu  Secret  Skill  unlock?

It  unlocks  by  having  him  get  hit  by  enemy.

Now  ask  the  question What  Go  1st  team  will  hit  your  Mortu  the  most?

Set  Up  A

i

Or

Set  Up  B

i

You  can  start  to  see  why  My  Mortu  is  on  a  team  called  Anti-Blender  team.