Context : Type Ibn supernovae ( SNe ) are rapidly-evolving and luminous ( 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 lightcurve of a fast Type Ibn .

Aims : We use 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 : Well-constrained lightcurve properties obtained from empirical models are compared with the literature .
We fit the pseudo-bolometric lightcurve with semi-analytical models powered by radioactive decay and CSM interaction .
The line profiles and emissivity of the prominent He i lines are modeled to study the formation of P-Cygni profiles and to estimate CSM properties .

Results : SN 2018bcc has a risetime to peak of 5.6 ^ { +0.2 } _ { -0.1 } days in the rest frame with a rising shape powerlaw index near 2 , and seems to be a typical rapidly-evolving Type Ibn SN .
The spectrum lacks signatures of SN-like ejecta and is dominated by over 15 He emission features at 20 days past peak , alongside Ca and Mg , all with V _ { \text { FWHM } } \sim 2000 \leavevmode \nobreak \text { km } \leavevmode \nobreak % \text { s } ^ { -1 } .
The luminous and rapidly-evolving lightcurve can be powered by CSM interaction but not by the decay of radioactive \@element [ ] [ 56 ] [ ] [ ] { \mathrm { Ni } } .
Modeling of the He i lines indicates 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 .