The Discussion Partner & Development Partner are often – but do not have to be – the same person. Both roles need to be undertaken by someone who is godly, wise, trusted, and can remain neutral.
For team members, the Discussion &/or Development Partner will usually be their line manager.
For senior leaders, the Discussion &/or Development Partner will often be the Chair of Trustees, an elder of the church or similar trusted leader. Your could ask someone from outside the organisation but this can reduce the relational benefit and make follow up / implementation of the Development Plan more challenging.