The graffiti writer credited with inventing the “bubble letter” style is Phase 2.
Phase 2 was the professional name of Michael Lawrence Marrow, a Bronx-born New York graffiti artist active mainly during the formative 1970s. In late 1972, he developed an early version of the rounded style known as bubble letters or “softies.” Its inflated, marshmallow-like forms made tags look bold, fluid and highly legible from a distance.
Other writers quickly adopted and modified the idea, helping bubble lettering become one of the most recognizable foundations of graffiti writing. Phase 2 did not stop at one design: he created named variations such as “bubble cloud,” “bubble drip” and “phasemagorical phantastic,” which added stars and other embellishments.
He is also credited with pioneering arrows in graffiti and later moved toward increasingly abstract, calligraphic work. In 1975, he joined United Graffiti Artists, a collective that helped bring New York aerosol art to wider public attention. Futura 2000 and Dondi were important innovators too, but they are not the standard answer to this specific origin question.