  Supernovae (SNe) Type Ibn are rapidly evolving and bright (M$_\text{R,peak}$
$\sim-19$) transients interacting with He-rich circumstellar material (CSM). SN
2018bcc, detected by the ZTF shortly after explosion, provides the best
constraints on the shape of the rising light curve (LC) of a fast Type Ibn.
Aims: We used the high-quality data set of SN 2018bcc to study observational
signatures of the class. Additionally, the powering mechanism of SN 2018bcc
offers insights into the debated progenitor connection of Type Ibn SNe.
Methods: We compared well-constrained LC properties obtained from empirical
models are compared with the literature. We fit the pseudo-bolometric LC with
semi-analytical models powered by radioactive decay and CSM interaction.
Finally, we modeled the line profiles and emissivity of the prominent He I
lines, in order to study the formation of P-Cygni profiles and estimate CSM
properties. Results: SN 2018bcc had a rise time to peak of $5.6^{+0.2}_{-0.1}$
days in the restframe with a rising shape power-law index close to 2, and seems
to be a typical rapidly evolving Type Ibn SN. The spectrum lacked signatures of
SN-like ejecta and was dominated by over 15 He emission features at 20 days
past peak, alongside Ca and Mg, all with V$_{\text{FWHM}} \sim
2000~\text{km}~\text{s}^{-1}$. The luminous and rapidly evolving LC could be
powered by CSM interaction but not by the decay of radioactive $^{56}$Ni.
Modeling of the He I lines indicated a dense and optically thick CSM that can
explain the P-Cygni profiles. Conclusions: Like other rapidly-evolving Type Ibn
SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by
shock interaction inside a dense and He-rich CSM. Its spectra do not support
the existence of two Type Ibn spectral classes. We also note the remarkable
observational match to pulsational pair instability (PPI) SN models.
