The main issue of HADES (High Acceptance Di-Electron Spectrometer) experiments is to study hot and dense nuclear matter by exploiting the di-electron decay channel of vector mesons produced in heavy ion collisions. The elementary reaction experiments performed in the last years with HADES provide selective information on different sources of di-electron emission and are therefore an indispensable complement to the heavy ion program. The proton-proton reaction has been measured at a 1.25 GeV kinetic energy with HADES. The energy, just below the threshold of Eta production, is well suited for the study of Delta Dalitz decay (Delta->Ne+e-) which is one of the important sources of di-electron pairs in the region of invariant masses below the contribution of vector mesons. This process is studied by leptonic inclusive ( pp->e+e-X) and exclusive (pp->e+e-pp) channel analyses. Meanwhile, the hadronic channel data analysis can also provide very precise and independent constraints on resonance production and decay mechanisms. In this contribution, the analyses of pp->pnpi+ and pp->pppi0 channels and comparison to detailed simulations (including Delta excitation in a one pion exchange model, non resonant pion production and final state interaction) will be presented. Detailed information on Delta decay can also be extracted from these data.