Restore user in DataBase that in master have mapped the child database but in child database doesn't have the user created I don't know why te following database arrive to this point but this is the situation

In master security you can see the defcon\axadmin user with DAXTest database mapped

But if you go to the database secury the user AXAdmin doesn't exist.

I can't edit any information about mapping of AXadmin user because throw the following error:

Also I can't delete this user in master because when I create again have the same mapping.

And if you check with te following sql:

use master
go
select * from syslogins where name='DEFCON\axadmin'
go 

use daxtest
go
select * from sysusers where name='DEFCON\axadmin'
go


You can see the user in master database but doesn't exist in DAXTest

I try to create in DAXTest 

CREATE USER [defcon\axadmin] FOR LOGIN [defcon\axadmin];


The user have another session start name

But if I try to edit

ALTER USER [DEFCON\axadmin] WITH LOGIN=[DEFCON\axadmin]


You can't edit the user because doesn't exist

Error:
Your login DEFCON\axadmin is sysadmin on the current server.

This means that it cannot be mapped to any database as DEFCON\axadmin user, it's already mapped and it's mapped as dbo in any database.

To see this just impersonate this login and ask for a user in that database:

use DAXTest;
execute as login = 'DEFCON\axadmin';
select user;
-- you'll see 'dbo' here
select select IS_SRVROLEMEMBER('sysadmin');
-- you'll get 1 here
revert;