The question you have posed is a subjective question. Therefore, the answer is subjective as well. What this means is that interpretation of anything is based solely upon a reader's or critic's interpretation of the posed question and their support of the answer.
That being said, William Blake could be considered the originator of the Romantic movement in English poetry given his artistic and imaginative works. Blake's works were, unfortunately, not recognized during his lifetime (like many authors). What one needs to understand is that periods, such as the Romantic period, were defined well after the movement began. Characteristics needed to be recognized as being followed by a greater whole so as to define a period's characteristics.
Consider the fact that many movements were created given the overall unhappiness of the previous period's work. This being said, the work of Blake did not follow the typical writing style of the period preceding the Romantic age (The Age of Reason). Blake's work was filled with complex symbols which showed his rebellion and imagination- both characteristics typical of the Romantic period. Reasoning was no longer the prominent theology of literature; instead, imagination and natural imagery were highlighted. The work of Blake epitomized both of these.
Blake depicted the Devil as the one who could fight the rules of society. This was, by far, a novel idea.
There can be much to suggest that Blake played a defining role in the Romantic Era. The most persuasive evidence would be that his ideas in both thought and art were prior to any other significant Romantic Poet of the time. Wordsworth develops his exploration of Romantic thematic concepts after Blake produces his work. Evidence suggests that a young Wordsworth learned from the older Blake in the former's early years of writing and thought. Another primary reason how Blake can be seen as an originator of the Romantic movement was that he was immersed in a complex relationship with the preceding intellectual movement of the time, Neoclassicism. With its strong emphasis on reason and scientific inquiry, Blake was driven to expose the limitations of reason, the complex nature of emotions, and the Romantic ideas of innocence and the transformation of society from what it is to what it can and should be. It is difficult to see other thinkers of this time period engaged with these issues as Blake was. Due to the fact that Blake spent much of his career wrestling with how his conception of Romanticism fit within a social and intellectual setting dominated by Neoclassicist thought, one can see Blake as a progenitor of the Romantic movement. The fact that he was misunderstood to a large extent and overlooked during his lifetime indicates that he might have been articulating the vision of a new movement, Romanticism, that had not been sensed at that time.