It repeatedly shows that Diina is super sketch, and then just leaves it as is. Summary of manga so far:
1. Isekai!
2. Diina comic relief x3
2. Hey look its a trap
3. Diina is lying and brainwashing everyone. I guess the golems' programming just 'forgot' about her existence. Seems legit
4. Goat man cries
5. Diina attempts to nuke an entire city of people. Result: "I totally want this dangerous girl beta tester of unknown age and origin as my advisor!" - mc
6. Diina drags mc away just as critical information is about to be revealed, mc makes no move to retrieve information because what is a manga without "intrigue" and buildup amirite
So at this point mc just lets Diina do whatever she wants even though it severely impedes knowing things such as "why is the demon lord attacking" or "why have I isekai'd here" because she's "helpful". Is mc constantly being brainwashed by the goddess? Is he/she just naturally passive af and doesn't care about these things? There is a limit to suspension of disbelief.