Regarding credit history, in spoken AmE we usually say 'perfect', 'good' or 'bad'. People with perfect credit often like to use the actual credit rating system's designation of 'AAA' (read as 'triple A') to describe their credit.
'I have perfect credit' or 'I have AAA credit.'
For your second question, I think your adjective 'reliable' is a good one to describe it, or just a short sentence stating that they pay back quickly.
A: I just lent XX $100, and now I wonder if I shouldn't have.
B: Don't worry, XX is always good about paying back money.
In AmE, 'reliable' is very commonly described in the phrase 'XX is always good about ...', like in:
'My son is always good about doing his homework without being told.' (I never have to tell him, he always does it himself)
'The landlord is always good about making repairs.' (The landlord reliably fixes broken things, broken things get fixed quickly)
(not a teacher, just a language lover)