  We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova
(SN) discovered during science validation of the Zwicky Transient Facility
(ZTF). ZTF18aalrxas was discovered while the optical emission was still rising
towards the initial cooling peak (0.7 mag over 2 days). Our observations
consist of multi-band (UV, optical) light-curves, and optical spectra spanning
from $\approx0.7$ d to $\approx180$ d past the explosion. We use a Monte-Carlo
based non-local thermodynamic equilibrium (NLTE) model, that simultanously
reproduces both the $\rm ^{56}Ni$ powered bolometric light curve and our
nebular spectrum. This model is used to constrain the synthesized radioactive
nickel mass (0.17 $\mathrm{M}_{\odot}$) and the total ejecta mass (1.7
$\mathrm{M}_{\odot}$) of the SN. The cooling emission is modeled using
semi-analytical extended envelope models to constrain the progenitor radius
($790-1050$ $\mathrm{R}_{\odot}$) at the time of explosion. Our nebular
spectrum shows signs of interaction with a dense circumstellar medium (CSM),
and this spetrum is modeled and analysed to constrain the amount of ejected
oxygen ($0.3-0.5$ $\mathrm{M}_{\odot}$) and the total hydrogen mass
($\approx0.15$ $\mathrm{M}_{\odot}$) in the envelope of the progenitor. The
oxygen mass of ZTF18aalrxas is consistent with a low ($12-13$
$\mathrm{M}_{\odot}$) Zero Age Main Sequence mass progenitor. The light curves
and spectra of ZTF18aalrxas are not consistent with massive single star SN Type
IIb progenitor models. The presence of an extended hydrogen envelope of low
mass, the presence of a dense CSM, the derived ejecta mass, and the late-time
oxygen emission can all be explained in a binary model scenario.
