How to get the AD account associated to a Proxy?
I think this is what you need. 

select 
    s.job_id
    ,s.name as [job_name]
    ,js.step_id
    ,js.step_name   
    ,js.subsystem
    ,js.proxy_id
    ,js.command
    , c.credential_identity
from sysjobs S
INNER JOIN sysjobsteps JS ON S.job_id = js.job_id
INNER JOIN sysproxies p ON js.proxy_id = p.proxy_id
INNER JOIN sys.credentials as c on c.credential_id = p.credential_id 


The proxy_id in sysjobsteps comes from sysproxies. You then get the account from credential_id in sys.credentials which is also available in sysproxies.