A few decades have passed since the identification of narrow-line Seyfert 1 (NLS1) galaxies as a subclass of active galactic nuclei (AGN). NLS1s show a Seyfert 1-like spectrum, but with emission line widths similar to Seyfert 2. Their observational properties suggest a combination of high accretion rates near the Eddington limit and low-mass black holes, indicating an early AGN stage. Although rare, jetted NLS1s have been discovered. Recently, seven sources with inverted radio spectra and extreme radio variability were identified among radio-quiet and radio-silent NLS1s. They show rapid flares at 37 GHz, increasing flux up to 9000-fold (Jy level) with e-folding timescales of a few hours. In quiescence and at lower frequencies, they reach only mJy levels. Relativistic jets would be the most likely cause, yet no jet traits were detected. So far, only their radio spectra are a common feature. I present these sources and a multi-epoch analysis to search for optical variability and periodicities using public data. A long-term comparison between radio and optical light curves and an optical spectral analysis were also conducted, revealing a scenario completely different from the radio band. The results show no strong optical variability and accretion at a few percent of the Eddington limit. Such radio variability has never been observed in any AGN, making this work key to understanding the phenomenon and possibly unveiling a new AGN population.